[libc-commits] [libc] [libc] implement localtime (PR #110363)

Зишан Мирза via libc-commits libc-commits at lists.llvm.org
Tue Oct 8 06:04:29 PDT 2024


https://github.com/zimirza updated https://github.com/llvm/llvm-project/pull/110363

>From 41b22383d62bccc5621435c6b117f7446b0cac99 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sat, 28 Sep 2024 15:50:29 +0200
Subject: [PATCH 01/24] [libc] implement localtime

This is an implementation of localtime.

Closes #107597 and #109892
---
 libc/config/baremetal/arm/entrypoints.txt   |  2 +
 libc/config/baremetal/riscv/entrypoints.txt |  2 +
 libc/config/linux/aarch64/entrypoints.txt   |  2 +
 libc/config/linux/riscv/entrypoints.txt     |  2 +
 libc/config/linux/x86_64/entrypoints.txt    |  2 +
 libc/newhdrgen/yaml/time.yaml               | 13 +++
 libc/spec/stdc.td                           | 13 +++
 libc/src/time/CMakeLists.txt                | 24 ++++++
 libc/src/time/localtime.cpp                 | 25 ++++++
 libc/src/time/localtime.h                   | 21 +++++
 libc/src/time/localtime_r.cpp               | 25 ++++++
 libc/src/time/localtime_r.h                 | 21 +++++
 libc/src/time/time_utils.cpp                | 45 ++++++++++-
 libc/src/time/time_utils.h                  | 35 ++++++++-
 libc/test/src/time/CMakeLists.txt           | 36 +++++++++
 libc/test/src/time/ctime_r_test.cpp         |  4 +-
 libc/test/src/time/ctime_test.cpp           |  4 +-
 libc/test/src/time/localtime_r_test.cpp     | 87 +++++++++++++++++++++
 libc/test/src/time/localtime_test.cpp       | 54 +++++++++++++
 19 files changed, 408 insertions(+), 9 deletions(-)
 create mode 100644 libc/src/time/localtime.cpp
 create mode 100644 libc/src/time/localtime.h
 create mode 100644 libc/src/time/localtime_r.cpp
 create mode 100644 libc/src/time/localtime_r.h
 create mode 100644 libc/test/src/time/localtime_r_test.cpp
 create mode 100644 libc/test/src/time/localtime_test.cpp

diff --git a/libc/config/baremetal/arm/entrypoints.txt b/libc/config/baremetal/arm/entrypoints.txt
index 68030f7f1775b5..7577fad2fde869 100644
--- a/libc/config/baremetal/arm/entrypoints.txt
+++ b/libc/config/baremetal/arm/entrypoints.txt
@@ -205,6 +205,8 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.time.asctime_r
     libc.src.time.ctime
     libc.src.time.ctime_r
+    libc.src.time.localtime
+    libc.src.time.localtime_r
     libc.src.time.difftime
     libc.src.time.gmtime
     libc.src.time.gmtime_r
diff --git a/libc/config/baremetal/riscv/entrypoints.txt b/libc/config/baremetal/riscv/entrypoints.txt
index 5894b591072ef0..b93aa334296ffb 100644
--- a/libc/config/baremetal/riscv/entrypoints.txt
+++ b/libc/config/baremetal/riscv/entrypoints.txt
@@ -201,6 +201,8 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.time.asctime_r
     libc.src.time.ctime
     libc.src.time.ctime_r
+    libc.src.time.localtime
+    libc.src.time.localtime_r
     libc.src.time.difftime
     libc.src.time.gmtime
     libc.src.time.gmtime_r
diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
index 64fbe1a250c0ba..d287bb0510a1fb 100644
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -950,6 +950,8 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.asctime_r
     libc.src.time.ctime
     libc.src.time.ctime_r
+    libc.src.time.localtime
+    libc.src.time.localtime_r
     libc.src.time.clock
     libc.src.time.clock_gettime
     libc.src.time.difftime
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
index ff3d821c664c5b..40a417bcf09b49 100644
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/libc/config/linux/riscv/entrypoints.txt
@@ -885,6 +885,8 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.asctime_r
     libc.src.time.ctime
     libc.src.time.ctime_r
+    libc.src.time.localtime
+    libc.src.time.localtime_r
     libc.src.time.clock
     libc.src.time.clock_gettime
     libc.src.time.difftime
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index dd658af3bfb674..3b63ce82417f4b 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -1005,6 +1005,8 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.asctime_r
     libc.src.time.ctime
     libc.src.time.ctime_r
+    libc.src.time.localtime
+    libc.src.time.localtime_r
     libc.src.time.clock
     libc.src.time.clock_gettime
     libc.src.time.difftime
diff --git a/libc/newhdrgen/yaml/time.yaml b/libc/newhdrgen/yaml/time.yaml
index 69b40bef3160dd..7198c19fce2bf0 100644
--- a/libc/newhdrgen/yaml/time.yaml
+++ b/libc/newhdrgen/yaml/time.yaml
@@ -37,6 +37,19 @@ functions:
     arguments:
       - type: const time_t *
       - type: char *
+  - name: localtime
+    standard:
+      - stdc
+    return_type: struct tm *
+    arguments:
+      - type: const time_t *
+  - name: localtime_r
+    standard:
+      - stdc
+    return_type: struct tm *
+    arguments:
+      - type: const time_t *
+      - type: struct tm *
   - name: clock
     standard:
       - stdc
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
index 7caf543748151a..9fd8d175f36be3 100644
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -1615,6 +1615,19 @@ def StdC : StandardSpec<"stdc"> {
                   ArgSpec<CharPtr>,
               ]
           >,
+          FunctionSpec<
+              "localtime",
+              RetValSpec<StructTmPtr>,
+              [ArgSpec<TimeTTypePtr>]
+          >,
+          FunctionSpec<
+              "localtime_r",
+              RetValSpec<StructTmPtr>,
+              [
+                  ArgSpec<TimeTTypePtr>,
+                  ArgSpec<StructTmPtr>,
+              ]
+          >,
           FunctionSpec<
               "clock",
               RetValSpec<ClockT>,
diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
index b3318e7ca87fa5..de2e128dd310c4 100644
--- a/libc/src/time/CMakeLists.txt
+++ b/libc/src/time/CMakeLists.txt
@@ -60,6 +60,30 @@ add_entrypoint_object(
     libc.include.time
 )
 
+add_entrypoint_object(
+  localtime
+  SRCS
+    localtime.cpp
+  HDRS
+    localtime.h
+  DEPENDS
+    .time_utils
+    libc.hdr.types.time_t
+    libc.include.time
+)
+
+add_entrypoint_object(
+  localtime_r
+  SRCS
+    localtime_r.cpp
+  HDRS
+    localtime_r.h
+  DEPENDS
+    .time_utils
+    libc.hdr.types.time_t
+    libc.include.time
+)
+
 add_entrypoint_object(
   difftime
   SRCS
diff --git a/libc/src/time/localtime.cpp b/libc/src/time/localtime.cpp
new file mode 100644
index 00000000000000..c08af2d8b26b78
--- /dev/null
+++ b/libc/src/time/localtime.cpp
@@ -0,0 +1,25 @@
+//===-- Implementation of localtime function ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/localtime.h"
+#include "src/__support/CPP/limits.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/time/time_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(struct tm *, localtime, (const time_t *t_ptr)) {
+  if (t_ptr == nullptr || *t_ptr > cpp::numeric_limits<int32_t>::max()) {
+      return nullptr;
+  }
+
+  return time_utils::localtime(t_ptr);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/localtime.h b/libc/src/time/localtime.h
new file mode 100644
index 00000000000000..648c8b755ddee9
--- /dev/null
+++ b/libc/src/time/localtime.h
@@ -0,0 +1,21 @@
+//===-- Implementation header of localtime ----------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_TIME_LOCALTIME_H
+#define LLVM_LIBC_SRC_TIME_LOCALTIME_H
+
+#include "src/__support/macros/config.h"
+#include <time.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+struct tm *localtime(const time_t *t_ptr);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_TIME_LOCALTIME_H
diff --git a/libc/src/time/localtime_r.cpp b/libc/src/time/localtime_r.cpp
new file mode 100644
index 00000000000000..6ff3fb3d4faa8c
--- /dev/null
+++ b/libc/src/time/localtime_r.cpp
@@ -0,0 +1,25 @@
+//===-- Implementation of localtime_r function ----------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/localtime_r.h"
+#include "src/__support/CPP/limits.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/time/time_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(struct tm *, localtime_r, (const time_t *t_ptr, struct tm *tm)) {
+  if (t_ptr == nullptr || *t_ptr > cpp::numeric_limits<int32_t>::max()) {
+      return nullptr;
+  }
+
+  return time_utils::localtime_internal(t_ptr, tm);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/localtime_r.h b/libc/src/time/localtime_r.h
new file mode 100644
index 00000000000000..d98b52180fa789
--- /dev/null
+++ b/libc/src/time/localtime_r.h
@@ -0,0 +1,21 @@
+//===-- Implementation header of localtime_r --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_TIME_LOCALTIME_R_H
+#define LLVM_LIBC_SRC_TIME_LOCALTIME_R_H
+
+#include "src/__support/macros/config.h"
+#include <time.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+struct tm *localtime_r(const time_t *t_ptr, struct tm *tm);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_TIME_LOCALTIME_R_H
diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 509cad8146df87..d00cc139471c8f 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <stdio.h>
 #include "src/time/time_utils.h"
 #include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
 #include "src/__support/common.h"
@@ -129,6 +130,25 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   if (years > INT_MAX || years < INT_MIN)
     return time_utils::out_of_range();
 
+  FILE *fp;
+  fp = fopen("/etc/timezone", "rb");
+  if (fp == NULL) {
+    return time_utils::out_of_range();
+  }
+
+  char timezone[128];
+  if (fgets(timezone, sizeof(timezone), fp) == NULL) {
+    return time_utils::out_of_range();
+  }
+
+  int offset;
+  if (internal::same_string(timezone, "UTC") == 0) {
+      offset = 0;
+  }
+  if (internal::same_string(timezone, "Europe/Berlin") == 0) {
+      offset = 2;
+  }
+
   // All the data (years, month and remaining days) was calculated from
   // March, 2000. Thus adjust the data to be from January, 1900.
   tm->tm_year = static_cast<int>(years + 2000 - TimeConstants::TIME_YEAR_BASE);
@@ -144,11 +164,32 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
                        TimeConstants::SECONDS_PER_MIN);
   tm->tm_sec =
       static_cast<int>(remainingSeconds % TimeConstants::SECONDS_PER_MIN);
-  // TODO(rtenneti): Need to handle timezone and update of tm_isdst.
-  tm->tm_isdst = 0;
+
+  if (offset == 0) {
+    tm->tm_isdst = 1;
+  } else {
+    tm->tm_isdst = 0;
+    tm->tm_hour += offset;
+  }
+
+  fclose(fp);
 
   return 0;
 }
 
+int calculate_dst(struct tm *tm) {
+  int sunday = tm->tm_mday - tm->tm_wday;
+
+  if (tm->tm_mon < 3 || tm->tm_mon > 11) {
+    return 0;
+  } else if (tm->tm_mon > 3 && tm->tm_mon < 11) {
+    return 1;
+  } else if (tm->tm_mon == 3) {
+      return sunday >= 8;
+  }
+
+  return sunday <= 0;
+}
+
 } // namespace time_utils
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 552ea925c1c7dc..97374ca5b17289 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -15,6 +15,7 @@
 #include "src/__support/macros/config.h"
 #include "src/errno/libc_errno.h"
 #include "src/time/mktime.h"
+#include "src/__support/CPP/limits.h"
 
 #include <stdint.h>
 
@@ -85,6 +86,7 @@ struct TimeConstants {
 // Update the "tm" structure's year, month, etc. members from seconds.
 // "total_seconds" is the number of seconds since January 1st, 1970.
 extern int64_t update_from_seconds(int64_t total_seconds, struct tm *tm);
+extern int calculate_dst(struct tm *tm);
 
 // TODO(michaelrj): move these functions to use ErrorOr instead of setting
 // errno. They always accompany a specific return value so we only need the one
@@ -156,11 +158,38 @@ LIBC_INLINE struct tm *gmtime_internal(const time_t *timer, struct tm *result) {
   return result;
 }
 
-// TODO: localtime is not yet implemented and a temporary solution is to
-//       use gmtime, https://github.com/llvm/llvm-project/issues/107597
 LIBC_INLINE struct tm *localtime(const time_t *t_ptr) {
   static struct tm result;
-  return time_utils::gmtime_internal(t_ptr, &result);
+  int64_t time = *t_ptr;
+
+  // Update the tm structure's year, month, day, etc. from seconds.
+  if (update_from_seconds(time, &result) < 0) {
+    out_of_range();
+    return nullptr;
+  }
+
+  int isdst = calculate_dst(&result);
+  result.tm_hour += isdst;
+  result.tm_isdst = isdst;
+
+  return &result;
+}
+
+LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr, struct tm *result) {
+  //time_t time = *t;
+  int64_t t = *t_ptr;
+
+  // Update the tm structure's year, month, day, etc. from seconds.
+  if (update_from_seconds(t, result) < 0) {
+    out_of_range();
+    return nullptr;
+  }
+
+  int isdst = calculate_dst(result);
+  result->tm_hour += isdst;
+  result->tm_isdst = isdst;
+
+  return result;
 }
 
 } // namespace time_utils
diff --git a/libc/test/src/time/CMakeLists.txt b/libc/test/src/time/CMakeLists.txt
index bba01f063fed27..e4f8944f6f3c62 100644
--- a/libc/test/src/time/CMakeLists.txt
+++ b/libc/test/src/time/CMakeLists.txt
@@ -66,6 +66,42 @@ add_libc_unittest(
     libc.src.time.time_utils
 )
 
+add_libc_unittest(
+  localtime_test
+  SUITE
+    libc_time_unittests
+  SRCS
+    localtime_test.cpp
+  HDRS
+    TmHelper.h
+    TmMatcher.h
+  CXX_STANDARD
+    20
+  DEPENDS
+    libc.include.time
+    libc.hdr.types.time_t
+    libc.src.time.localtime
+    libc.src.time.time_utils
+)
+
+add_libc_unittest(
+  localtime_r_test
+  SUITE
+    libc_time_unittests
+  SRCS
+    localtime_r_test.cpp
+  HDRS
+    TmHelper.h
+    TmMatcher.h
+  CXX_STANDARD
+    20
+  DEPENDS
+    libc.include.time
+    libc.hdr.types.time_t
+    libc.src.time.localtime_r
+    libc.src.time.time_utils
+)
+
 add_libc_test(
   clock_gettime_test
   SUITE
diff --git a/libc/test/src/time/ctime_r_test.cpp b/libc/test/src/time/ctime_r_test.cpp
index 9ce6f75f754849..e0aa32e28178dc 100644
--- a/libc/test/src/time/ctime_r_test.cpp
+++ b/libc/test/src/time/ctime_r_test.cpp
@@ -35,7 +35,7 @@ TEST(LlvmLibcCtimeR, ValidUnixTimestamp0) {
   // 1970-01-01 00:00:00. Test with a valid buffer size.
   t = 0;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Thu Jan  1 00:00:00 1970\n", result);
+  ASSERT_STREQ("Thu Jan  1 01:00:00 1970\n", result);
 }
 
 TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
@@ -45,7 +45,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
   // 2038-01-19 03:14:07. Test with a valid buffer size.
   t = 2147483647;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Tue Jan 19 03:14:07 2038\n", result);
+  ASSERT_STREQ("Tue Jan 19 04:14:07 2038\n", result);
 }
 
 TEST(LlvmLibcCtimeR, InvalidArgument) {
diff --git a/libc/test/src/time/ctime_test.cpp b/libc/test/src/time/ctime_test.cpp
index 7ec71bb1e4ed1e..554b72d23fd4de 100644
--- a/libc/test/src/time/ctime_test.cpp
+++ b/libc/test/src/time/ctime_test.cpp
@@ -22,7 +22,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp0) {
   char *result;
   t = 0;
   result = LIBC_NAMESPACE::ctime(&t);
-  ASSERT_STREQ("Thu Jan  1 00:00:00 1970\n", result);
+  ASSERT_STREQ("Thu Jan  1 01:00:00 1970\n", result);
 }
 
 TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
@@ -30,7 +30,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
   char *result;
   t = 2147483647;
   result = LIBC_NAMESPACE::ctime(&t);
-  ASSERT_STREQ("Tue Jan 19 03:14:07 2038\n", result);
+  ASSERT_STREQ("Tue Jan 19 04:14:07 2038\n", result);
 }
 
 TEST(LlvmLibcCtime, InvalidArgument) {
diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
new file mode 100644
index 00000000000000..83848b53fc7d54
--- /dev/null
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -0,0 +1,87 @@
+//===-- Unittests for localtime_r -----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/errno/libc_errno.h"
+#include "src/time/localtime_r.h"
+#include "test/UnitTest/Test.h"
+#include "test/src/time/TmHelper.h"
+
+TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
+  const time_t t_ptr = 1;
+  static struct tm input = (struct tm) {
+      .tm_sec = 0,
+      .tm_min = 0,
+      .tm_hour = 0,
+      .tm_mday = 0,
+      .tm_mon = 0,
+      .tm_year = 0,
+      .tm_wday = 0,
+      .tm_yday = 0,
+      .tm_isdst = 0
+  };
+  struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
+  ASSERT_EQ(70, result->tm_year);
+  ASSERT_EQ(0, result->tm_mon);
+  ASSERT_EQ(1, result->tm_mday);
+  ASSERT_EQ(2, result->tm_hour);
+  ASSERT_EQ(0, result->tm_min);
+  ASSERT_EQ(1, result->tm_sec);
+  ASSERT_EQ(4, result->tm_wday);
+  ASSERT_EQ(0, result->tm_yday);
+  ASSERT_EQ(0, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
+  time_t t_ptr = 2147483647;
+  static struct tm input = (struct tm) {
+      .tm_sec = 0,
+      .tm_min = 0,
+      .tm_hour = 0,
+      .tm_mday = 0,
+      .tm_mon = 0,
+      .tm_year = 0,
+      .tm_wday = 0,
+      .tm_yday = 0,
+      .tm_isdst = 0
+  };
+  struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
+  ASSERT_EQ(138, result->tm_year);
+  ASSERT_EQ(0, result->tm_mon);
+  ASSERT_EQ(19, result->tm_mday);
+  ASSERT_EQ(5, result->tm_hour);
+  ASSERT_EQ(14, result->tm_min);
+  ASSERT_EQ(7, result->tm_sec);
+  ASSERT_EQ(2, result->tm_wday);
+  ASSERT_EQ(18, result->tm_yday);
+  ASSERT_EQ(0, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32IntDst) {
+  time_t t_ptr = 1627225465;
+  static struct tm input = (struct tm) {
+      .tm_sec = 0,
+      .tm_min = 0,
+      .tm_hour = 0,
+      .tm_mday = 0,
+      .tm_mon = 0,
+      .tm_year = 0,
+      .tm_wday = 0,
+      .tm_yday = 0,
+      .tm_isdst = 0
+  };
+  struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(18, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}
diff --git a/libc/test/src/time/localtime_test.cpp b/libc/test/src/time/localtime_test.cpp
new file mode 100644
index 00000000000000..ba068dbf632d02
--- /dev/null
+++ b/libc/test/src/time/localtime_test.cpp
@@ -0,0 +1,54 @@
+//===-- Unittests for localtime -------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/errno/libc_errno.h"
+#include "src/time/localtime.h"
+#include "test/UnitTest/Test.h"
+#include "test/src/time/TmHelper.h"
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestamp0) {
+  const time_t t_ptr = 0;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(70, result->tm_year);
+  ASSERT_EQ(0, result->tm_mon);
+  ASSERT_EQ(1, result->tm_mday);
+  ASSERT_EQ(2, result->tm_hour);
+  ASSERT_EQ(0, result->tm_min);
+  ASSERT_EQ(0, result->tm_sec);
+  ASSERT_EQ(4, result->tm_wday);
+  ASSERT_EQ(0, result->tm_yday);
+  ASSERT_EQ(0, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestamp32Int) {
+  time_t t_ptr = 2147483647;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(138, result->tm_year);
+  ASSERT_EQ(0, result->tm_mon);
+  ASSERT_EQ(19, result->tm_mday);
+  ASSERT_EQ(5, result->tm_hour);
+  ASSERT_EQ(14, result->tm_min);
+  ASSERT_EQ(7, result->tm_sec);
+  ASSERT_EQ(2, result->tm_wday);
+  ASSERT_EQ(18, result->tm_yday);
+  ASSERT_EQ(0, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestamp32IntDst) {
+  time_t t_ptr = 1627225465;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(18, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}

>From 4280c136c7fa56643a83b6184be894bfca259588 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sat, 28 Sep 2024 16:02:12 +0200
Subject: [PATCH 02/24] [libc] implement localtime

format code with clang-format
---
 libc/src/time/localtime.cpp             |  2 +-
 libc/src/time/localtime_r.cpp           |  5 ++-
 libc/src/time/time_utils.cpp            |  8 ++--
 libc/src/time/time_utils.h              |  7 +--
 libc/test/src/time/localtime_r_test.cpp | 60 +++++++++++--------------
 5 files changed, 39 insertions(+), 43 deletions(-)

diff --git a/libc/src/time/localtime.cpp b/libc/src/time/localtime.cpp
index c08af2d8b26b78..48c8d4a6c6b3f4 100644
--- a/libc/src/time/localtime.cpp
+++ b/libc/src/time/localtime.cpp
@@ -16,7 +16,7 @@ namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(struct tm *, localtime, (const time_t *t_ptr)) {
   if (t_ptr == nullptr || *t_ptr > cpp::numeric_limits<int32_t>::max()) {
-      return nullptr;
+    return nullptr;
   }
 
   return time_utils::localtime(t_ptr);
diff --git a/libc/src/time/localtime_r.cpp b/libc/src/time/localtime_r.cpp
index 6ff3fb3d4faa8c..64a107a729132f 100644
--- a/libc/src/time/localtime_r.cpp
+++ b/libc/src/time/localtime_r.cpp
@@ -14,9 +14,10 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-LLVM_LIBC_FUNCTION(struct tm *, localtime_r, (const time_t *t_ptr, struct tm *tm)) {
+LLVM_LIBC_FUNCTION(struct tm *, localtime_r,
+                   (const time_t *t_ptr, struct tm *tm)) {
   if (t_ptr == nullptr || *t_ptr > cpp::numeric_limits<int32_t>::max()) {
-      return nullptr;
+    return nullptr;
   }
 
   return time_utils::localtime_internal(t_ptr, tm);
diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index d00cc139471c8f..9a4aacb8f86f85 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -6,11 +6,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include <stdio.h>
 #include "src/time/time_utils.h"
 #include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
+#include <stdio.h>
 
 namespace LIBC_NAMESPACE_DECL {
 namespace time_utils {
@@ -143,10 +143,10 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
 
   int offset;
   if (internal::same_string(timezone, "UTC") == 0) {
-      offset = 0;
+    offset = 0;
   }
   if (internal::same_string(timezone, "Europe/Berlin") == 0) {
-      offset = 2;
+    offset = 2;
   }
 
   // All the data (years, month and remaining days) was calculated from
@@ -185,7 +185,7 @@ int calculate_dst(struct tm *tm) {
   } else if (tm->tm_mon > 3 && tm->tm_mon < 11) {
     return 1;
   } else if (tm->tm_mon == 3) {
-      return sunday >= 8;
+    return sunday >= 8;
   }
 
   return sunday <= 0;
diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 97374ca5b17289..87e0e0bae767c8 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -11,11 +11,11 @@
 
 #include <stddef.h> // For size_t.
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
 #include "src/errno/libc_errno.h"
 #include "src/time/mktime.h"
-#include "src/__support/CPP/limits.h"
 
 #include <stdint.h>
 
@@ -175,8 +175,9 @@ LIBC_INLINE struct tm *localtime(const time_t *t_ptr) {
   return &result;
 }
 
-LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr, struct tm *result) {
-  //time_t time = *t;
+LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr,
+                                          struct tm *result) {
+  // time_t time = *t;
   int64_t t = *t_ptr;
 
   // Update the tm structure's year, month, day, etc. from seconds.
diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index 83848b53fc7d54..edc7fed030aa9d 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -13,17 +13,15 @@
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
   const time_t t_ptr = 1;
-  static struct tm input = (struct tm) {
-      .tm_sec = 0,
-      .tm_min = 0,
-      .tm_hour = 0,
-      .tm_mday = 0,
-      .tm_mon = 0,
-      .tm_year = 0,
-      .tm_wday = 0,
-      .tm_yday = 0,
-      .tm_isdst = 0
-  };
+  static struct tm input = (struct tm){.tm_sec = 0,
+                                       .tm_min = 0,
+                                       .tm_hour = 0,
+                                       .tm_mday = 0,
+                                       .tm_mon = 0,
+                                       .tm_year = 0,
+                                       .tm_wday = 0,
+                                       .tm_yday = 0,
+                                       .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
   ASSERT_EQ(70, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
@@ -38,17 +36,15 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
   time_t t_ptr = 2147483647;
-  static struct tm input = (struct tm) {
-      .tm_sec = 0,
-      .tm_min = 0,
-      .tm_hour = 0,
-      .tm_mday = 0,
-      .tm_mon = 0,
-      .tm_year = 0,
-      .tm_wday = 0,
-      .tm_yday = 0,
-      .tm_isdst = 0
-  };
+  static struct tm input = (struct tm){.tm_sec = 0,
+                                       .tm_min = 0,
+                                       .tm_hour = 0,
+                                       .tm_mday = 0,
+                                       .tm_mon = 0,
+                                       .tm_year = 0,
+                                       .tm_wday = 0,
+                                       .tm_yday = 0,
+                                       .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
   ASSERT_EQ(138, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
@@ -63,17 +59,15 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32IntDst) {
   time_t t_ptr = 1627225465;
-  static struct tm input = (struct tm) {
-      .tm_sec = 0,
-      .tm_min = 0,
-      .tm_hour = 0,
-      .tm_mday = 0,
-      .tm_mon = 0,
-      .tm_year = 0,
-      .tm_wday = 0,
-      .tm_yday = 0,
-      .tm_isdst = 0
-  };
+  static struct tm input = (struct tm){.tm_sec = 0,
+                                       .tm_min = 0,
+                                       .tm_hour = 0,
+                                       .tm_mday = 0,
+                                       .tm_mon = 0,
+                                       .tm_year = 0,
+                                       .tm_wday = 0,
+                                       .tm_yday = 0,
+                                       .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
   ASSERT_EQ(121, result->tm_year);
   ASSERT_EQ(6, result->tm_mon);

>From 15a5bef567cd7535a4ace2dcee0da4bddb97b954 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sat, 28 Sep 2024 16:05:42 +0200
Subject: [PATCH 03/24] [libc] implement localtime

fix: dst
---
 libc/src/time/time_utils.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 9a4aacb8f86f85..5cc735734111a7 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -166,9 +166,9 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
       static_cast<int>(remainingSeconds % TimeConstants::SECONDS_PER_MIN);
 
   if (offset == 0) {
-    tm->tm_isdst = 1;
-  } else {
     tm->tm_isdst = 0;
+  } else {
+    tm->tm_isdst = 1;
     tm->tm_hour += offset;
   }
 

>From 9970dfefba4073a55d5bc73be43c03a5a12d6c7b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sat, 28 Sep 2024 16:16:06 +0200
Subject: [PATCH 04/24] [libc] implement localtime

fix: dst
---
 libc/src/time/time_utils.h              | 1 -
 libc/test/src/time/localtime_r_test.cpp | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 87e0e0bae767c8..0be7f114d370f9 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -187,7 +187,6 @@ LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr,
   }
 
   int isdst = calculate_dst(result);
-  result->tm_hour += isdst;
   result->tm_isdst = isdst;
 
   return result;
diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index edc7fed030aa9d..3c5d3c64e6bd9f 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -72,7 +72,7 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32IntDst) {
   ASSERT_EQ(121, result->tm_year);
   ASSERT_EQ(6, result->tm_mon);
   ASSERT_EQ(25, result->tm_mday);
-  ASSERT_EQ(18, result->tm_hour);
+  ASSERT_EQ(17, result->tm_hour);
   ASSERT_EQ(4, result->tm_min);
   ASSERT_EQ(25, result->tm_sec);
   ASSERT_EQ(0, result->tm_wday);

>From 6dee10b36c82cc3483e281c717c749a624479f3c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sat, 28 Sep 2024 16:20:15 +0200
Subject: [PATCH 05/24] [libc] implement localtime

update test for `localtime_r`
---
 libc/test/src/time/localtime_r_test.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index 3c5d3c64e6bd9f..805db50751b777 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -12,7 +12,7 @@
 #include "test/src/time/TmHelper.h"
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
-  const time_t t_ptr = 1;
+  const time_t t_ptr = 0;
   static struct tm input = (struct tm){.tm_sec = 0,
                                        .tm_min = 0,
                                        .tm_hour = 0,
@@ -28,7 +28,7 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
   ASSERT_EQ(1, result->tm_mday);
   ASSERT_EQ(2, result->tm_hour);
   ASSERT_EQ(0, result->tm_min);
-  ASSERT_EQ(1, result->tm_sec);
+  ASSERT_EQ(0, result->tm_sec);
   ASSERT_EQ(4, result->tm_wday);
   ASSERT_EQ(0, result->tm_yday);
   ASSERT_EQ(0, result->tm_isdst);

>From 429d13aa01a6d9efbf23e16cbf31b5d89009dfd1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sat, 28 Sep 2024 16:22:35 +0200
Subject: [PATCH 06/24] [libc] implement localtime

remove unnecessary code
---
 libc/src/time/time_utils.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 0be7f114d370f9..494ebc0bedf276 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -177,7 +177,6 @@ LIBC_INLINE struct tm *localtime(const time_t *t_ptr) {
 
 LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr,
                                           struct tm *result) {
-  // time_t time = *t;
   int64_t t = *t_ptr;
 
   // Update the tm structure's year, month, day, etc. from seconds.

>From 6dc9f4c6b22cde2393742e215d4c725d89907843 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sat, 28 Sep 2024 16:34:52 +0200
Subject: [PATCH 07/24] [libc] implement localtime

fix: dst
---
 libc/src/time/time_utils.h            | 1 -
 libc/test/src/time/localtime_test.cpp | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 494ebc0bedf276..6faf8f37172f6c 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -169,7 +169,6 @@ LIBC_INLINE struct tm *localtime(const time_t *t_ptr) {
   }
 
   int isdst = calculate_dst(&result);
-  result.tm_hour += isdst;
   result.tm_isdst = isdst;
 
   return &result;
diff --git a/libc/test/src/time/localtime_test.cpp b/libc/test/src/time/localtime_test.cpp
index ba068dbf632d02..4143e7065596ff 100644
--- a/libc/test/src/time/localtime_test.cpp
+++ b/libc/test/src/time/localtime_test.cpp
@@ -45,7 +45,7 @@ TEST(LlvmLibcLocaltime, ValidUnixTimestamp32IntDst) {
   ASSERT_EQ(121, result->tm_year);
   ASSERT_EQ(6, result->tm_mon);
   ASSERT_EQ(25, result->tm_mday);
-  ASSERT_EQ(18, result->tm_hour);
+  ASSERT_EQ(17, result->tm_hour);
   ASSERT_EQ(4, result->tm_min);
   ASSERT_EQ(25, result->tm_sec);
   ASSERT_EQ(0, result->tm_wday);

>From 054d070f0cf4e11bc649c420200894cb15d49ed7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 16:30:21 +0200
Subject: [PATCH 08/24] localtime_s is for windows only, implement localtime_s
 on gnu/linux for compatibility reasons

---
 libc/config/baremetal/arm/entrypoints.txt   |  1 +
 libc/config/baremetal/riscv/entrypoints.txt |  1 +
 libc/config/linux/aarch64/entrypoints.txt   |  1 +
 libc/config/linux/riscv/entrypoints.txt     |  1 +
 libc/config/linux/x86_64/entrypoints.txt    |  1 +
 libc/docs/c23.rst                           |  1 +
 libc/docs/date_and_time.rst                 |  2 +
 libc/newhdrgen/yaml/time.yaml               |  7 +++
 libc/spec/stdc.td                           |  8 ++++
 libc/src/time/CMakeLists.txt                | 12 +++++
 libc/src/time/localtime.cpp                 |  8 ++--
 libc/src/time/localtime_r.cpp               |  9 +---
 libc/src/time/localtime_r.h                 |  2 +-
 libc/src/time/localtime_s.cpp               | 21 +++++++++
 libc/src/time/localtime_s.h                 | 22 +++++++++
 libc/src/time/time_utils.h                  | 37 +++++++++++++--
 libc/test/src/time/CMakeLists.txt           | 19 ++++++++
 libc/test/src/time/ctime_r_test.cpp         |  4 +-
 libc/test/src/time/localtime_r_test.cpp     | 50 +++++++++-----------
 libc/test/src/time/localtime_s_test.cpp     | 52 +++++++++++++++++++++
 20 files changed, 213 insertions(+), 46 deletions(-)
 create mode 100644 libc/src/time/localtime_s.cpp
 create mode 100644 libc/src/time/localtime_s.h
 create mode 100644 libc/test/src/time/localtime_s_test.cpp

diff --git a/libc/config/baremetal/arm/entrypoints.txt b/libc/config/baremetal/arm/entrypoints.txt
index 7577fad2fde869..0a905d50f8ba25 100644
--- a/libc/config/baremetal/arm/entrypoints.txt
+++ b/libc/config/baremetal/arm/entrypoints.txt
@@ -207,6 +207,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.time.ctime_r
     libc.src.time.localtime
     libc.src.time.localtime_r
+    libc.src.time.localtime_s
     libc.src.time.difftime
     libc.src.time.gmtime
     libc.src.time.gmtime_r
diff --git a/libc/config/baremetal/riscv/entrypoints.txt b/libc/config/baremetal/riscv/entrypoints.txt
index b93aa334296ffb..b9ea143c770262 100644
--- a/libc/config/baremetal/riscv/entrypoints.txt
+++ b/libc/config/baremetal/riscv/entrypoints.txt
@@ -203,6 +203,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.time.ctime_r
     libc.src.time.localtime
     libc.src.time.localtime_r
+    libc.src.time.localtime_s
     libc.src.time.difftime
     libc.src.time.gmtime
     libc.src.time.gmtime_r
diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
index d287bb0510a1fb..a4b430f1e84ea4 100644
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -952,6 +952,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.ctime_r
     libc.src.time.localtime
     libc.src.time.localtime_r
+    libc.src.time.localtime_s
     libc.src.time.clock
     libc.src.time.clock_gettime
     libc.src.time.difftime
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
index 40a417bcf09b49..18f6e6418a7af9 100644
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/libc/config/linux/riscv/entrypoints.txt
@@ -887,6 +887,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.ctime_r
     libc.src.time.localtime
     libc.src.time.localtime_r
+    libc.src.time.localtime_s
     libc.src.time.clock
     libc.src.time.clock_gettime
     libc.src.time.difftime
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index 3b63ce82417f4b..5b9b0dea1b31ae 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -1007,6 +1007,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.ctime_r
     libc.src.time.localtime
     libc.src.time.localtime_r
+    libc.src.time.localtime_s
     libc.src.time.clock
     libc.src.time.clock_gettime
     libc.src.time.difftime
diff --git a/libc/docs/c23.rst b/libc/docs/c23.rst
index b9a2424307f84b..9f0515bda5d581 100644
--- a/libc/docs/c23.rst
+++ b/libc/docs/c23.rst
@@ -152,6 +152,7 @@ Additions:
 
   * gmtime_r
   * localtime_r
+  * localtime_s
   * timegm
   * timespec_getres
   * strftime conversion specifiers
diff --git a/libc/docs/date_and_time.rst b/libc/docs/date_and_time.rst
index b745a3b416f802..ab7c7c3de68677 100644
--- a/libc/docs/date_and_time.rst
+++ b/libc/docs/date_and_time.rst
@@ -89,6 +89,8 @@ Implementation Status
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
 | localtime_r         |         |         |         |                 |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
+| localtime_s         |         |         |         |                 |         |         |         |         |         |         |         |         |
++---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
 | mktime              | |check| | |check| |         |     |check|     |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
 | nanosleep           | |check| | |check| |         |     |check|     |         |         |         |         |         |         |         |         |
diff --git a/libc/newhdrgen/yaml/time.yaml b/libc/newhdrgen/yaml/time.yaml
index 7198c19fce2bf0..b0311b64165472 100644
--- a/libc/newhdrgen/yaml/time.yaml
+++ b/libc/newhdrgen/yaml/time.yaml
@@ -50,6 +50,13 @@ functions:
     arguments:
       - type: const time_t *
       - type: struct tm *
+  - name: localtime_s
+    standard:
+      - stdc
+    return_type: struct tm *
+    arguments:
+      - type: const time_t *
+      - type: struct tm *
   - name: clock
     standard:
       - stdc
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
index 9fd8d175f36be3..5546a3afcab9f7 100644
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -1628,6 +1628,14 @@ def StdC : StandardSpec<"stdc"> {
                   ArgSpec<StructTmPtr>,
               ]
           >,
+          FunctionSpec<
+              "localtime_s",
+              RetValSpec<StructTmPtr>,
+              [
+                  ArgSpec<TimeTTypePtr>,
+                  ArgSpec<StructTmPtr>,
+              ]
+          >,
           FunctionSpec<
               "clock",
               RetValSpec<ClockT>,
diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
index de2e128dd310c4..57b229d029c92e 100644
--- a/libc/src/time/CMakeLists.txt
+++ b/libc/src/time/CMakeLists.txt
@@ -84,6 +84,18 @@ add_entrypoint_object(
     libc.include.time
 )
 
+add_entrypoint_object(
+  localtime_s
+  SRCS
+    localtime_s.cpp
+  HDRS
+    localtime_s.h
+  DEPENDS
+    .time_utils
+    libc.hdr.types.time_t
+    libc.include.time
+)
+
 add_entrypoint_object(
   difftime
   SRCS
diff --git a/libc/src/time/localtime.cpp b/libc/src/time/localtime.cpp
index 48c8d4a6c6b3f4..42ba562e0a561c 100644
--- a/libc/src/time/localtime.cpp
+++ b/libc/src/time/localtime.cpp
@@ -6,16 +6,18 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/time/localtime.h"
+#include "localtime.h"
 #include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
-#include "src/time/time_utils.h"
+#include "time_utils.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
+using LIBC_NAMESPACE::time_utils::TimeConstants;
+
 LLVM_LIBC_FUNCTION(struct tm *, localtime, (const time_t *t_ptr)) {
-  if (t_ptr == nullptr || *t_ptr > cpp::numeric_limits<int32_t>::max()) {
+  if (t_ptr == nullptr) {
     return nullptr;
   }
 
diff --git a/libc/src/time/localtime_r.cpp b/libc/src/time/localtime_r.cpp
index 64a107a729132f..0af5414930327b 100644
--- a/libc/src/time/localtime_r.cpp
+++ b/libc/src/time/localtime_r.cpp
@@ -7,7 +7,6 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/time/localtime_r.h"
-#include "src/__support/CPP/limits.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
 #include "src/time/time_utils.h"
@@ -15,12 +14,8 @@
 namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(struct tm *, localtime_r,
-                   (const time_t *t_ptr, struct tm *tm)) {
-  if (t_ptr == nullptr || *t_ptr > cpp::numeric_limits<int32_t>::max()) {
-    return nullptr;
-  }
-
-  return time_utils::localtime_internal(t_ptr, tm);
+                   (const time_t *t_ptr, struct tm *input)) {
+  return time_utils::localtime_internal(t_ptr, input);
 }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/localtime_r.h b/libc/src/time/localtime_r.h
index d98b52180fa789..553811d6ffdc2b 100644
--- a/libc/src/time/localtime_r.h
+++ b/libc/src/time/localtime_r.h
@@ -14,7 +14,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-struct tm *localtime_r(const time_t *t_ptr, struct tm *tm);
+struct tm *localtime_r(const time_t *t_ptr, struct tm *input);
 
 } // namespace LIBC_NAMESPACE_DECL
 
diff --git a/libc/src/time/localtime_s.cpp b/libc/src/time/localtime_s.cpp
new file mode 100644
index 00000000000000..e49ff49810819f
--- /dev/null
+++ b/libc/src/time/localtime_s.cpp
@@ -0,0 +1,21 @@
+//===-- Implementation of localtime_s function ----------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/localtime_s.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/time/time_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+// windows only, implemented in gnu/linux for compatibility reasons
+LLVM_LIBC_FUNCTION(int, localtime_s, (const time_t *t_ptr, struct tm *input)) {
+  return time_utils::localtime_s(t_ptr, input);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/localtime_s.h b/libc/src/time/localtime_s.h
new file mode 100644
index 00000000000000..4f69cb0014d507
--- /dev/null
+++ b/libc/src/time/localtime_s.h
@@ -0,0 +1,22 @@
+//===-- Implementation header of localtime_s --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_TIME_LOCALTIME_S_H
+#define LLVM_LIBC_SRC_TIME_LOCALTIME_S_H
+
+#include "src/__support/macros/config.h"
+#include <time.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+// windows only, implemented in gnu/linux for compatibility reasons
+int localtime_s(const time_t *t_ptr, struct tm *input);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_TIME_LOCALTIME_S_H
diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 6faf8f37172f6c..78bfa069471231 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -175,19 +175,46 @@ LIBC_INLINE struct tm *localtime(const time_t *t_ptr) {
 }
 
 LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr,
-                                          struct tm *result) {
+                                          struct tm *input) {
+  static struct tm result;
   int64_t t = *t_ptr;
 
+  result.tm_sec = input->tm_sec;
+  result.tm_min = input->tm_min;
+  result.tm_hour = input->tm_hour;
+  result.tm_mday = input->tm_mday;
+  result.tm_mon = input->tm_mon;
+  result.tm_year = input->tm_year;
+  result.tm_wday = input->tm_wday;
+  result.tm_yday = input->tm_yday;
+  result.tm_isdst = input->tm_isdst;
+
   // Update the tm structure's year, month, day, etc. from seconds.
-  if (update_from_seconds(t, result) < 0) {
+  if (update_from_seconds(t, &result) < 0) {
     out_of_range();
     return nullptr;
   }
 
-  int isdst = calculate_dst(result);
-  result->tm_isdst = isdst;
+  int isdst = calculate_dst(&result);
+  result.tm_isdst = isdst;
 
-  return result;
+  return &result;
+}
+
+// for windows only, implemented on gnu/linux for compatibility reasons
+LIBC_INLINE int localtime_s(const time_t *t_ptr, struct tm *input) {
+  static struct tm *result = localtime_internal(t_ptr, input);
+  time_t t = LIBC_NAMESPACE::mktime(result);
+
+  if (*t_ptr < t) {
+    return -1;
+  }
+
+  if (*t_ptr > t) {
+    return 1;
+  }
+
+  return 0;
 }
 
 } // namespace time_utils
diff --git a/libc/test/src/time/CMakeLists.txt b/libc/test/src/time/CMakeLists.txt
index e4f8944f6f3c62..54e078cff83c15 100644
--- a/libc/test/src/time/CMakeLists.txt
+++ b/libc/test/src/time/CMakeLists.txt
@@ -102,6 +102,25 @@ add_libc_unittest(
     libc.src.time.time_utils
 )
 
+add_libc_unittest(
+  localtime_s_test
+  SUITE
+    libc_time_unittests
+  SRCS
+    localtime_s_test.cpp
+  HDRS
+    TmHelper.h
+    TmMatcher.h
+  CXX_STANDARD
+    20
+  DEPENDS
+    libc.include.time
+    libc.hdr.types.time_t
+    libc.src.time.mktime
+    libc.src.time.localtime_s
+    libc.src.time.time_utils
+)
+
 add_libc_test(
   clock_gettime_test
   SUITE
diff --git a/libc/test/src/time/ctime_r_test.cpp b/libc/test/src/time/ctime_r_test.cpp
index e0aa32e28178dc..b03e8cecd2e9df 100644
--- a/libc/test/src/time/ctime_r_test.cpp
+++ b/libc/test/src/time/ctime_r_test.cpp
@@ -35,7 +35,7 @@ TEST(LlvmLibcCtimeR, ValidUnixTimestamp0) {
   // 1970-01-01 00:00:00. Test with a valid buffer size.
   t = 0;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Thu Jan  1 01:00:00 1970\n", result);
+  ASSERT_STREQ("Thu Jan  1 02:00:00 1970\n", result);
 }
 
 TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
@@ -45,7 +45,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
   // 2038-01-19 03:14:07. Test with a valid buffer size.
   t = 2147483647;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Tue Jan 19 04:14:07 2038\n", result);
+  ASSERT_STREQ("Tue Jan 19 05:14:07 2038\n", result);
 }
 
 TEST(LlvmLibcCtimeR, InvalidArgument) {
diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index 805db50751b777..ba46835e81f77a 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -6,22 +6,16 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/errno/libc_errno.h"
 #include "src/time/localtime_r.h"
+#include "src/time/time_utils.h"
 #include "test/UnitTest/Test.h"
 #include "test/src/time/TmHelper.h"
 
+using LIBC_NAMESPACE::time_utils::TimeConstants;
+
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
+  struct tm input;
   const time_t t_ptr = 0;
-  static struct tm input = (struct tm){.tm_sec = 0,
-                                       .tm_min = 0,
-                                       .tm_hour = 0,
-                                       .tm_mday = 0,
-                                       .tm_mon = 0,
-                                       .tm_year = 0,
-                                       .tm_wday = 0,
-                                       .tm_yday = 0,
-                                       .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
   ASSERT_EQ(70, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
@@ -36,15 +30,15 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
   time_t t_ptr = 2147483647;
-  static struct tm input = (struct tm){.tm_sec = 0,
-                                       .tm_min = 0,
-                                       .tm_hour = 0,
-                                       .tm_mday = 0,
-                                       .tm_mon = 0,
-                                       .tm_year = 0,
-                                       .tm_wday = 0,
-                                       .tm_yday = 0,
-                                       .tm_isdst = 0};
+  struct tm input = (struct tm){.tm_sec = 0,
+                                .tm_min = 0,
+                                .tm_hour = 0,
+                                .tm_mday = 0,
+                                .tm_mon = 0,
+                                .tm_year = 0,
+                                .tm_wday = 0,
+                                .tm_yday = 0,
+                                .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
   ASSERT_EQ(138, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
@@ -59,15 +53,15 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32IntDst) {
   time_t t_ptr = 1627225465;
-  static struct tm input = (struct tm){.tm_sec = 0,
-                                       .tm_min = 0,
-                                       .tm_hour = 0,
-                                       .tm_mday = 0,
-                                       .tm_mon = 0,
-                                       .tm_year = 0,
-                                       .tm_wday = 0,
-                                       .tm_yday = 0,
-                                       .tm_isdst = 0};
+  struct tm input = (struct tm){.tm_sec = 0,
+                                .tm_min = 0,
+                                .tm_hour = 0,
+                                .tm_mday = 0,
+                                .tm_mon = 0,
+                                .tm_year = 0,
+                                .tm_wday = 0,
+                                .tm_yday = 0,
+                                .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
   ASSERT_EQ(121, result->tm_year);
   ASSERT_EQ(6, result->tm_mon);
diff --git a/libc/test/src/time/localtime_s_test.cpp b/libc/test/src/time/localtime_s_test.cpp
new file mode 100644
index 00000000000000..04d5d79c810792
--- /dev/null
+++ b/libc/test/src/time/localtime_s_test.cpp
@@ -0,0 +1,52 @@
+//===-- Unittests for localtime_s -----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/localtime_s.h"
+#include "src/time/mktime.h"
+#include "src/time/time_utils.h"
+#include "test/UnitTest/Test.h"
+#include "test/src/time/TmHelper.h"
+
+using LIBC_NAMESPACE::time_utils::TimeConstants;
+
+TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp0) {
+  struct tm input;
+  const time_t t_ptr = 0;
+  int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
+  ASSERT_EQ(-1, result);
+}
+
+TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32Int) {
+  time_t t_ptr = 2147483647;
+  static struct tm input = (struct tm){.tm_sec = 0,
+                                       .tm_min = 0,
+                                       .tm_hour = 0,
+                                       .tm_mday = 0,
+                                       .tm_mon = 0,
+                                       .tm_year = 0,
+                                       .tm_wday = 0,
+                                       .tm_yday = 0,
+                                       .tm_isdst = 0};
+  int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
+  ASSERT_EQ(1, result);
+}
+
+TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32IntDst) {
+  time_t t_ptr = 1627225465;
+  static struct tm input = (struct tm){.tm_sec = 0,
+                                       .tm_min = 0,
+                                       .tm_hour = 0,
+                                       .tm_mday = 0,
+                                       .tm_mon = 0,
+                                       .tm_year = 0,
+                                       .tm_wday = 0,
+                                       .tm_yday = 0,
+                                       .tm_isdst = 0};
+  int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
+  ASSERT_EQ(1, result);
+}

>From a09d3af60719db6bdebb9eb20c5ce01fe36ab8f7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 16:37:58 +0200
Subject: [PATCH 09/24] fix: localtime_s

---
 libc/newhdrgen/yaml/time.yaml | 2 +-
 libc/spec/stdc.td             | 2 +-
 libc/src/time/CMakeLists.txt  | 1 +
 3 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/libc/newhdrgen/yaml/time.yaml b/libc/newhdrgen/yaml/time.yaml
index b0311b64165472..9c249e21fc8d82 100644
--- a/libc/newhdrgen/yaml/time.yaml
+++ b/libc/newhdrgen/yaml/time.yaml
@@ -53,7 +53,7 @@ functions:
   - name: localtime_s
     standard:
       - stdc
-    return_type: struct tm *
+    return_type: int
     arguments:
       - type: const time_t *
       - type: struct tm *
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
index 5546a3afcab9f7..d6186281b76a3f 100644
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -1630,7 +1630,7 @@ def StdC : StandardSpec<"stdc"> {
           >,
           FunctionSpec<
               "localtime_s",
-              RetValSpec<StructTmPtr>,
+              RetValSpec<IntType>,
               [
                   ArgSpec<TimeTTypePtr>,
                   ArgSpec<StructTmPtr>,
diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
index 57b229d029c92e..e983ccf6359e93 100644
--- a/libc/src/time/CMakeLists.txt
+++ b/libc/src/time/CMakeLists.txt
@@ -94,6 +94,7 @@ add_entrypoint_object(
     .time_utils
     libc.hdr.types.time_t
     libc.include.time
+    libc.time.mktime
 )
 
 add_entrypoint_object(

>From 897c8209c85e30567244703e861376ac2d504cc9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 17:58:41 +0200
Subject: [PATCH 10/24] fix: path for mktime

---
 libc/src/time/CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
index e983ccf6359e93..096d6bcc7dd8ef 100644
--- a/libc/src/time/CMakeLists.txt
+++ b/libc/src/time/CMakeLists.txt
@@ -94,7 +94,7 @@ add_entrypoint_object(
     .time_utils
     libc.hdr.types.time_t
     libc.include.time
-    libc.time.mktime
+    libc.src.time.mktime
 )
 
 add_entrypoint_object(

>From 31b3f2483f11e68659f043fdae2dd12003bba00d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 19:38:42 +0200
Subject: [PATCH 11/24] fix: localtime_s and dst function

---
 libc/src/time/time_utils.cpp            | 18 ++++--
 libc/src/time/time_utils.h              | 45 +++++++--------
 libc/test/src/time/localtime_s_test.cpp | 77 ++++++++++++++++++++++---
 3 files changed, 103 insertions(+), 37 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 5cc735734111a7..35fbc02c17e679 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -177,18 +177,24 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   return 0;
 }
 
-int calculate_dst(struct tm *tm) {
-  int sunday = tm->tm_mday - tm->tm_wday;
+void set_dst(struct tm *tm) {
+  int dst;
+  int sunday;
+
+  dst = 0;
+  sunday = tm->tm_mday - tm->tm_wday;
 
   if (tm->tm_mon < 3 || tm->tm_mon > 11) {
-    return 0;
+    dst = 0;
   } else if (tm->tm_mon > 3 && tm->tm_mon < 11) {
-    return 1;
+    dst = 1;
   } else if (tm->tm_mon == 3) {
-    return sunday >= 8;
+    dst = sunday >= 8;
+  } else {
+    dst = sunday <= 0;
   }
 
-  return sunday <= 0;
+  tm->tm_isdst = dst;
 }
 
 } // namespace time_utils
diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 78bfa069471231..51442493e19d87 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -86,7 +86,7 @@ struct TimeConstants {
 // Update the "tm" structure's year, month, etc. members from seconds.
 // "total_seconds" is the number of seconds since January 1st, 1970.
 extern int64_t update_from_seconds(int64_t total_seconds, struct tm *tm);
-extern int calculate_dst(struct tm *tm);
+extern void set_dst(struct tm *tm);
 
 // TODO(michaelrj): move these functions to use ErrorOr instead of setting
 // errno. They always accompany a specific return value so we only need the one
@@ -168,51 +168,48 @@ LIBC_INLINE struct tm *localtime(const time_t *t_ptr) {
     return nullptr;
   }
 
-  int isdst = calculate_dst(&result);
-  result.tm_isdst = isdst;
+  set_dst(&result);
 
   return &result;
 }
 
 LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr,
                                           struct tm *input) {
-  static struct tm result;
   int64_t t = *t_ptr;
 
-  result.tm_sec = input->tm_sec;
-  result.tm_min = input->tm_min;
-  result.tm_hour = input->tm_hour;
-  result.tm_mday = input->tm_mday;
-  result.tm_mon = input->tm_mon;
-  result.tm_year = input->tm_year;
-  result.tm_wday = input->tm_wday;
-  result.tm_yday = input->tm_yday;
-  result.tm_isdst = input->tm_isdst;
-
   // Update the tm structure's year, month, day, etc. from seconds.
-  if (update_from_seconds(t, &result) < 0) {
+  if (update_from_seconds(t, input) < 0) {
     out_of_range();
     return nullptr;
   }
 
-  int isdst = calculate_dst(&result);
-  result.tm_isdst = isdst;
+  set_dst(input);
 
-  return &result;
+  return input;
 }
 
 // for windows only, implemented on gnu/linux for compatibility reasons
 LIBC_INLINE int localtime_s(const time_t *t_ptr, struct tm *input) {
-  static struct tm *result = localtime_internal(t_ptr, input);
-  time_t t = LIBC_NAMESPACE::mktime(result);
+  if (input == NULL)
+    return -1;
+
+  if ((*t_ptr < 0 || *t_ptr > cpp::numeric_limits<int64_t>::max()) && input != NULL) {
+    // setting values to -1 for compatibility reasons
+    // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s
+    input->tm_sec = -1;
+    input->tm_min = -1;
+    input->tm_hour = -1;
+    input->tm_mday = -1;
+    input->tm_mon = -1;
+    input->tm_year = -1;
+    input->tm_wday = -1;
+    input->tm_yday = -1;
+    input->tm_isdst = -1;
 
-  if (*t_ptr < t) {
     return -1;
   }
 
-  if (*t_ptr > t) {
-    return 1;
-  }
+  localtime_internal(t_ptr, input);
 
   return 0;
 }
diff --git a/libc/test/src/time/localtime_s_test.cpp b/libc/test/src/time/localtime_s_test.cpp
index 04d5d79c810792..4a9d1d5dba5414 100644
--- a/libc/test/src/time/localtime_s_test.cpp
+++ b/libc/test/src/time/localtime_s_test.cpp
@@ -15,15 +15,33 @@
 using LIBC_NAMESPACE::time_utils::TimeConstants;
 
 TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp0) {
-  struct tm input;
-  const time_t t_ptr = 0;
+  struct tm input = (struct tm){.tm_sec = 0,
+                                .tm_min = 0,
+                                .tm_hour = 0,
+                                .tm_mday = 0,
+                                .tm_mon = 0,
+                                .tm_year = 0,
+                                .tm_wday = 0,
+                                .tm_yday = 0,
+                                .tm_isdst = 0};
+  time_t t_ptr = 0;
   int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
-  ASSERT_EQ(-1, result);
+  ASSERT_EQ(0, result);
+
+  ASSERT_EQ(70, input.tm_year);
+  ASSERT_EQ(0, input.tm_mon);
+  ASSERT_EQ(1, input.tm_mday);
+  ASSERT_EQ(2, input.tm_hour);
+  ASSERT_EQ(0, input.tm_min);
+  ASSERT_EQ(0, input.tm_sec);
+  ASSERT_EQ(4, input.tm_wday);
+  ASSERT_EQ(0, input.tm_yday);
+  ASSERT_EQ(0, input.tm_isdst);
 }
 
 TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32Int) {
   time_t t_ptr = 2147483647;
-  static struct tm input = (struct tm){.tm_sec = 0,
+  struct tm input = (struct tm){.tm_sec = 0,
                                        .tm_min = 0,
                                        .tm_hour = 0,
                                        .tm_mday = 0,
@@ -33,12 +51,22 @@ TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32Int) {
                                        .tm_yday = 0,
                                        .tm_isdst = 0};
   int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
-  ASSERT_EQ(1, result);
+  ASSERT_EQ(0, result);
+
+  ASSERT_EQ(138, input.tm_year);
+  ASSERT_EQ(0, input.tm_mon);
+  ASSERT_EQ(19, input.tm_mday);
+  ASSERT_EQ(5, input.tm_hour);
+  ASSERT_EQ(14, input.tm_min);
+  ASSERT_EQ(7, input.tm_sec);
+  ASSERT_EQ(2, input.tm_wday);
+  ASSERT_EQ(18, input.tm_yday);
+  ASSERT_EQ(0, input.tm_isdst);
 }
 
 TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32IntDst) {
   time_t t_ptr = 1627225465;
-  static struct tm input = (struct tm){.tm_sec = 0,
+  struct tm input = (struct tm){.tm_sec = 0,
                                        .tm_min = 0,
                                        .tm_hour = 0,
                                        .tm_mday = 0,
@@ -48,5 +76,40 @@ TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32IntDst) {
                                        .tm_yday = 0,
                                        .tm_isdst = 0};
   int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
-  ASSERT_EQ(1, result);
+  ASSERT_EQ(0, result);
+
+  ASSERT_EQ(121, input.tm_year);
+  ASSERT_EQ(6, input.tm_mon);
+  ASSERT_EQ(25, input.tm_mday);
+  ASSERT_EQ(17, input.tm_hour);
+  ASSERT_EQ(4, input.tm_min);
+  ASSERT_EQ(25, input.tm_sec);
+  ASSERT_EQ(0, input.tm_wday);
+  ASSERT_EQ(205, input.tm_yday);
+  ASSERT_EQ(1, input.tm_isdst);
+}
+
+TEST(LlvmLibcLocaltimeS, InvalidUnixTimestamp) {
+  time_t t_ptr = -1;
+  struct tm input = (struct tm){.tm_sec = 0,
+                                       .tm_min = 0,
+                                       .tm_hour = 0,
+                                       .tm_mday = 0,
+                                       .tm_mon = 0,
+                                       .tm_year = 0,
+                                       .tm_wday = 0,
+                                       .tm_yday = 0,
+                                       .tm_isdst = 0};
+  int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
+  ASSERT_EQ(-1, result);
+
+  ASSERT_EQ(-1, input.tm_year);
+  ASSERT_EQ(-1, input.tm_mon);
+  ASSERT_EQ(-1, input.tm_mday);
+  ASSERT_EQ(-1, input.tm_hour);
+  ASSERT_EQ(-1, input.tm_min);
+  ASSERT_EQ(-1, input.tm_sec);
+  ASSERT_EQ(-1, input.tm_wday);
+  ASSERT_EQ(-1, input.tm_yday);
+  ASSERT_EQ(-1, input.tm_isdst);
 }

>From 0dbcfd3a8387dd69d83e2ea5a7701bcb4cf1af74 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 19:42:14 +0200
Subject: [PATCH 12/24] added tests for localtime_r

---
 libc/test/src/time/localtime_r_test.cpp | 35 ++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index ba46835e81f77a..1a3e4be11e6c36 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -15,8 +15,19 @@ using LIBC_NAMESPACE::time_utils::TimeConstants;
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
   struct tm input;
-  const time_t t_ptr = 0;
+  time_t t_ptr = 0;
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
+
+  ASSERT_EQ(70, input.tm_year);
+  ASSERT_EQ(0, input.tm_mon);
+  ASSERT_EQ(1, input.tm_mday);
+  ASSERT_EQ(2, input.tm_hour);
+  ASSERT_EQ(0, input.tm_min);
+  ASSERT_EQ(0, input.tm_sec);
+  ASSERT_EQ(4, input.tm_wday);
+  ASSERT_EQ(0, input.tm_yday);
+  ASSERT_EQ(0, input.tm_isdst);
+
   ASSERT_EQ(70, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
   ASSERT_EQ(1, result->tm_mday);
@@ -40,6 +51,17 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
                                 .tm_yday = 0,
                                 .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
+
+  ASSERT_EQ(138, input.tm_year);
+  ASSERT_EQ(0, input.tm_mon);
+  ASSERT_EQ(19, input.tm_mday);
+  ASSERT_EQ(5, input.tm_hour);
+  ASSERT_EQ(14, input.tm_min);
+  ASSERT_EQ(7, input.tm_sec);
+  ASSERT_EQ(2, input.tm_wday);
+  ASSERT_EQ(18, input.tm_yday);
+  ASSERT_EQ(0, input.tm_isdst);
+
   ASSERT_EQ(138, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
   ASSERT_EQ(19, result->tm_mday);
@@ -63,6 +85,17 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32IntDst) {
                                 .tm_yday = 0,
                                 .tm_isdst = 0};
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
+
+  ASSERT_EQ(121, input.tm_year);
+  ASSERT_EQ(6, input.tm_mon);
+  ASSERT_EQ(25, input.tm_mday);
+  ASSERT_EQ(17, input.tm_hour);
+  ASSERT_EQ(4, input.tm_min);
+  ASSERT_EQ(25, input.tm_sec);
+  ASSERT_EQ(0, input.tm_wday);
+  ASSERT_EQ(205, input.tm_yday);
+  ASSERT_EQ(1, input.tm_isdst);
+
   ASSERT_EQ(121, result->tm_year);
   ASSERT_EQ(6, result->tm_mon);
   ASSERT_EQ(25, result->tm_mday);

>From a86b91f93c89228b2b8b9c574df34c0d7795b582 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 19:43:47 +0200
Subject: [PATCH 13/24] format code with clang-format

---
 libc/src/time/time_utils.h              |  3 +-
 libc/test/src/time/localtime_s_test.cpp | 48 ++++++++++++-------------
 2 files changed, 26 insertions(+), 25 deletions(-)

diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 51442493e19d87..929487a16567c5 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -193,7 +193,8 @@ LIBC_INLINE int localtime_s(const time_t *t_ptr, struct tm *input) {
   if (input == NULL)
     return -1;
 
-  if ((*t_ptr < 0 || *t_ptr > cpp::numeric_limits<int64_t>::max()) && input != NULL) {
+  if ((*t_ptr < 0 || *t_ptr > cpp::numeric_limits<int64_t>::max()) &&
+      input != NULL) {
     // setting values to -1 for compatibility reasons
     // https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/localtime-s-localtime32-s-localtime64-s
     input->tm_sec = -1;
diff --git a/libc/test/src/time/localtime_s_test.cpp b/libc/test/src/time/localtime_s_test.cpp
index 4a9d1d5dba5414..cf97945a8fe4a0 100644
--- a/libc/test/src/time/localtime_s_test.cpp
+++ b/libc/test/src/time/localtime_s_test.cpp
@@ -42,14 +42,14 @@ TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp0) {
 TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32Int) {
   time_t t_ptr = 2147483647;
   struct tm input = (struct tm){.tm_sec = 0,
-                                       .tm_min = 0,
-                                       .tm_hour = 0,
-                                       .tm_mday = 0,
-                                       .tm_mon = 0,
-                                       .tm_year = 0,
-                                       .tm_wday = 0,
-                                       .tm_yday = 0,
-                                       .tm_isdst = 0};
+                                .tm_min = 0,
+                                .tm_hour = 0,
+                                .tm_mday = 0,
+                                .tm_mon = 0,
+                                .tm_year = 0,
+                                .tm_wday = 0,
+                                .tm_yday = 0,
+                                .tm_isdst = 0};
   int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
   ASSERT_EQ(0, result);
 
@@ -67,14 +67,14 @@ TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32Int) {
 TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32IntDst) {
   time_t t_ptr = 1627225465;
   struct tm input = (struct tm){.tm_sec = 0,
-                                       .tm_min = 0,
-                                       .tm_hour = 0,
-                                       .tm_mday = 0,
-                                       .tm_mon = 0,
-                                       .tm_year = 0,
-                                       .tm_wday = 0,
-                                       .tm_yday = 0,
-                                       .tm_isdst = 0};
+                                .tm_min = 0,
+                                .tm_hour = 0,
+                                .tm_mday = 0,
+                                .tm_mon = 0,
+                                .tm_year = 0,
+                                .tm_wday = 0,
+                                .tm_yday = 0,
+                                .tm_isdst = 0};
   int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
   ASSERT_EQ(0, result);
 
@@ -92,14 +92,14 @@ TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32IntDst) {
 TEST(LlvmLibcLocaltimeS, InvalidUnixTimestamp) {
   time_t t_ptr = -1;
   struct tm input = (struct tm){.tm_sec = 0,
-                                       .tm_min = 0,
-                                       .tm_hour = 0,
-                                       .tm_mday = 0,
-                                       .tm_mon = 0,
-                                       .tm_year = 0,
-                                       .tm_wday = 0,
-                                       .tm_yday = 0,
-                                       .tm_isdst = 0};
+                                .tm_min = 0,
+                                .tm_hour = 0,
+                                .tm_mday = 0,
+                                .tm_mon = 0,
+                                .tm_year = 0,
+                                .tm_wday = 0,
+                                .tm_yday = 0,
+                                .tm_isdst = 0};
   int result = LIBC_NAMESPACE::localtime_s(&t_ptr, &input);
   ASSERT_EQ(-1, result);
 

>From 2da5c6c89473ef2f8f4a1041b48aec47fc4b0a9b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 19:59:14 +0200
Subject: [PATCH 14/24] refactor: timezone

---
 libc/src/time/time_utils.cpp | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 35fbc02c17e679..90fec7cf730286 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -130,24 +130,22 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   if (years > INT_MAX || years < INT_MIN)
     return time_utils::out_of_range();
 
+  char timezone[128];
+
   FILE *fp;
   fp = fopen("/etc/timezone", "rb");
   if (fp == NULL) {
+    // TODO: implement getting timezone from `TZ` environment variable and
+    // storing the value in `timezone`
+  } else if (fgets(timezone, sizeof(timezone), fp) == NULL)
     return time_utils::out_of_range();
-  }
-
-  char timezone[128];
-  if (fgets(timezone, sizeof(timezone), fp) == NULL) {
-    return time_utils::out_of_range();
-  }
 
   int offset;
-  if (internal::same_string(timezone, "UTC") == 0) {
+  // TODO: Add more timezones
+  if (internal::same_string(timezone, "UTC") == 0)
     offset = 0;
-  }
-  if (internal::same_string(timezone, "Europe/Berlin") == 0) {
+  if (internal::same_string(timezone, "Europe/Berlin") == 0)
     offset = 2;
-  }
 
   // All the data (years, month and remaining days) was calculated from
   // March, 2000. Thus adjust the data to be from January, 1900.
@@ -167,9 +165,11 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
 
   if (offset == 0) {
     tm->tm_isdst = 0;
-  } else {
+  } else if (offset > 0) {
     tm->tm_isdst = 1;
     tm->tm_hour += offset;
+  } else {
+    tm->tm_isdst = -1;
   }
 
   fclose(fp);

>From 5ce3de549fb78807ad6941b83a9c521ea4108a75 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 20:04:11 +0200
Subject: [PATCH 15/24] removing mktime as dependency to localtime_s

---
 libc/src/time/CMakeLists.txt      | 1 -
 libc/test/src/time/CMakeLists.txt | 1 -
 2 files changed, 2 deletions(-)

diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
index 096d6bcc7dd8ef..57b229d029c92e 100644
--- a/libc/src/time/CMakeLists.txt
+++ b/libc/src/time/CMakeLists.txt
@@ -94,7 +94,6 @@ add_entrypoint_object(
     .time_utils
     libc.hdr.types.time_t
     libc.include.time
-    libc.src.time.mktime
 )
 
 add_entrypoint_object(
diff --git a/libc/test/src/time/CMakeLists.txt b/libc/test/src/time/CMakeLists.txt
index 54e078cff83c15..376bdcac836bab 100644
--- a/libc/test/src/time/CMakeLists.txt
+++ b/libc/test/src/time/CMakeLists.txt
@@ -116,7 +116,6 @@ add_libc_unittest(
   DEPENDS
     libc.include.time
     libc.hdr.types.time_t
-    libc.src.time.mktime
     libc.src.time.localtime_s
     libc.src.time.time_utils
 )

>From 03e50941780e1ee4095d2596a257a58847c81d3a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 20:11:41 +0200
Subject: [PATCH 16/24] revert ctime tests

---
 libc/test/src/time/ctime_test.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libc/test/src/time/ctime_test.cpp b/libc/test/src/time/ctime_test.cpp
index 554b72d23fd4de..7ec71bb1e4ed1e 100644
--- a/libc/test/src/time/ctime_test.cpp
+++ b/libc/test/src/time/ctime_test.cpp
@@ -22,7 +22,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp0) {
   char *result;
   t = 0;
   result = LIBC_NAMESPACE::ctime(&t);
-  ASSERT_STREQ("Thu Jan  1 01:00:00 1970\n", result);
+  ASSERT_STREQ("Thu Jan  1 00:00:00 1970\n", result);
 }
 
 TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
@@ -30,7 +30,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
   char *result;
   t = 2147483647;
   result = LIBC_NAMESPACE::ctime(&t);
-  ASSERT_STREQ("Tue Jan 19 04:14:07 2038\n", result);
+  ASSERT_STREQ("Tue Jan 19 03:14:07 2038\n", result);
 }
 
 TEST(LlvmLibcCtime, InvalidArgument) {

>From 60e07cdf36446d3ce3ee0f7d7cbb7fedafadbea5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 20:12:54 +0200
Subject: [PATCH 17/24] revert ctime_r tests

---
 libc/test/src/time/ctime_r_test.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libc/test/src/time/ctime_r_test.cpp b/libc/test/src/time/ctime_r_test.cpp
index b03e8cecd2e9df..9ce6f75f754849 100644
--- a/libc/test/src/time/ctime_r_test.cpp
+++ b/libc/test/src/time/ctime_r_test.cpp
@@ -35,7 +35,7 @@ TEST(LlvmLibcCtimeR, ValidUnixTimestamp0) {
   // 1970-01-01 00:00:00. Test with a valid buffer size.
   t = 0;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Thu Jan  1 02:00:00 1970\n", result);
+  ASSERT_STREQ("Thu Jan  1 00:00:00 1970\n", result);
 }
 
 TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
@@ -45,7 +45,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
   // 2038-01-19 03:14:07. Test with a valid buffer size.
   t = 2147483647;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Tue Jan 19 05:14:07 2038\n", result);
+  ASSERT_STREQ("Tue Jan 19 03:14:07 2038\n", result);
 }
 
 TEST(LlvmLibcCtimeR, InvalidArgument) {

>From a8074d447926614f10e3d91795f5f53a4508caa0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 21:18:10 +0200
Subject: [PATCH 18/24] fix: daylight saving time

---
 libc/src/time/time_utils.cpp            | 18 +++++++-----------
 libc/src/time/time_utils.h              |  4 ----
 libc/test/src/time/ctime_r_test.cpp     |  8 ++++----
 libc/test/src/time/ctime_test.cpp       |  4 ++--
 libc/test/src/time/localtime_r_test.cpp |  8 ++++----
 libc/test/src/time/localtime_s_test.cpp |  4 ++--
 libc/test/src/time/localtime_test.cpp   |  4 ++--
 7 files changed, 21 insertions(+), 29 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 90fec7cf730286..df55090185f3e2 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -140,12 +140,11 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   } else if (fgets(timezone, sizeof(timezone), fp) == NULL)
     return time_utils::out_of_range();
 
-  int offset;
+  // UTC = 0
+  int offset = 0;
   // TODO: Add more timezones
-  if (internal::same_string(timezone, "UTC") == 0)
-    offset = 0;
   if (internal::same_string(timezone, "Europe/Berlin") == 0)
-    offset = 2;
+    offset = 1;
 
   // All the data (years, month and remaining days) was calculated from
   // March, 2000. Thus adjust the data to be from January, 1900.
@@ -163,14 +162,11 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   tm->tm_sec =
       static_cast<int>(remainingSeconds % TimeConstants::SECONDS_PER_MIN);
 
-  if (offset == 0) {
-    tm->tm_isdst = 0;
-  } else if (offset > 0) {
-    tm->tm_isdst = 1;
-    tm->tm_hour += offset;
-  } else {
-    tm->tm_isdst = -1;
+  set_dst(tm);
+  if (tm->tm_isdst > 0) {
+    tm->tm_hour += 1;
   }
+  tm->tm_hour += offset;
 
   fclose(fp);
 
diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index 929487a16567c5..aba8c7f2c14920 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -168,8 +168,6 @@ LIBC_INLINE struct tm *localtime(const time_t *t_ptr) {
     return nullptr;
   }
 
-  set_dst(&result);
-
   return &result;
 }
 
@@ -183,8 +181,6 @@ LIBC_INLINE struct tm *localtime_internal(const time_t *t_ptr,
     return nullptr;
   }
 
-  set_dst(input);
-
   return input;
 }
 
diff --git a/libc/test/src/time/ctime_r_test.cpp b/libc/test/src/time/ctime_r_test.cpp
index 9ce6f75f754849..5ca4b5b1ea228d 100644
--- a/libc/test/src/time/ctime_r_test.cpp
+++ b/libc/test/src/time/ctime_r_test.cpp
@@ -32,20 +32,20 @@ TEST(LlvmLibcCtimeR, ValidUnixTimestamp0) {
   char buffer[TimeConstants::ASCTIME_BUFFER_SIZE];
   time_t t;
   char *result;
-  // 1970-01-01 00:00:00. Test with a valid buffer size.
+  // 1970-01-01 01:00:00. Test with a valid buffer size.
   t = 0;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Thu Jan  1 00:00:00 1970\n", result);
+  ASSERT_STREQ("Thu Jan  1 01:00:00 1970\n", result);
 }
 
 TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
   char buffer[TimeConstants::ASCTIME_BUFFER_SIZE];
   time_t t;
   char *result;
-  // 2038-01-19 03:14:07. Test with a valid buffer size.
+  // 2038-01-19 04:14:07. Test with a valid buffer size.
   t = 2147483647;
   result = LIBC_NAMESPACE::ctime_r(&t, buffer);
-  ASSERT_STREQ("Tue Jan 19 03:14:07 2038\n", result);
+  ASSERT_STREQ("Tue Jan 19 04:14:07 2038\n", result);
 }
 
 TEST(LlvmLibcCtimeR, InvalidArgument) {
diff --git a/libc/test/src/time/ctime_test.cpp b/libc/test/src/time/ctime_test.cpp
index 7ec71bb1e4ed1e..554b72d23fd4de 100644
--- a/libc/test/src/time/ctime_test.cpp
+++ b/libc/test/src/time/ctime_test.cpp
@@ -22,7 +22,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp0) {
   char *result;
   t = 0;
   result = LIBC_NAMESPACE::ctime(&t);
-  ASSERT_STREQ("Thu Jan  1 00:00:00 1970\n", result);
+  ASSERT_STREQ("Thu Jan  1 01:00:00 1970\n", result);
 }
 
 TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
@@ -30,7 +30,7 @@ TEST(LlvmLibcCtime, ValidUnixTimestamp32Int) {
   char *result;
   t = 2147483647;
   result = LIBC_NAMESPACE::ctime(&t);
-  ASSERT_STREQ("Tue Jan 19 03:14:07 2038\n", result);
+  ASSERT_STREQ("Tue Jan 19 04:14:07 2038\n", result);
 }
 
 TEST(LlvmLibcCtime, InvalidArgument) {
diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index 1a3e4be11e6c36..03b885fa4aa96b 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -21,7 +21,7 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
   ASSERT_EQ(70, input.tm_year);
   ASSERT_EQ(0, input.tm_mon);
   ASSERT_EQ(1, input.tm_mday);
-  ASSERT_EQ(2, input.tm_hour);
+  ASSERT_EQ(1, input.tm_hour);
   ASSERT_EQ(0, input.tm_min);
   ASSERT_EQ(0, input.tm_sec);
   ASSERT_EQ(4, input.tm_wday);
@@ -31,7 +31,7 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
   ASSERT_EQ(70, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
   ASSERT_EQ(1, result->tm_mday);
-  ASSERT_EQ(2, result->tm_hour);
+  ASSERT_EQ(1, result->tm_hour);
   ASSERT_EQ(0, result->tm_min);
   ASSERT_EQ(0, result->tm_sec);
   ASSERT_EQ(4, result->tm_wday);
@@ -55,7 +55,7 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
   ASSERT_EQ(138, input.tm_year);
   ASSERT_EQ(0, input.tm_mon);
   ASSERT_EQ(19, input.tm_mday);
-  ASSERT_EQ(5, input.tm_hour);
+  ASSERT_EQ(4, input.tm_hour);
   ASSERT_EQ(14, input.tm_min);
   ASSERT_EQ(7, input.tm_sec);
   ASSERT_EQ(2, input.tm_wday);
@@ -65,7 +65,7 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
   ASSERT_EQ(138, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
   ASSERT_EQ(19, result->tm_mday);
-  ASSERT_EQ(5, result->tm_hour);
+  ASSERT_EQ(4, result->tm_hour);
   ASSERT_EQ(14, result->tm_min);
   ASSERT_EQ(7, result->tm_sec);
   ASSERT_EQ(2, result->tm_wday);
diff --git a/libc/test/src/time/localtime_s_test.cpp b/libc/test/src/time/localtime_s_test.cpp
index cf97945a8fe4a0..aeb658fe6fd871 100644
--- a/libc/test/src/time/localtime_s_test.cpp
+++ b/libc/test/src/time/localtime_s_test.cpp
@@ -31,7 +31,7 @@ TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp0) {
   ASSERT_EQ(70, input.tm_year);
   ASSERT_EQ(0, input.tm_mon);
   ASSERT_EQ(1, input.tm_mday);
-  ASSERT_EQ(2, input.tm_hour);
+  ASSERT_EQ(1, input.tm_hour);
   ASSERT_EQ(0, input.tm_min);
   ASSERT_EQ(0, input.tm_sec);
   ASSERT_EQ(4, input.tm_wday);
@@ -56,7 +56,7 @@ TEST(LlvmLibcLocaltimeS, ValidUnixTimestamp32Int) {
   ASSERT_EQ(138, input.tm_year);
   ASSERT_EQ(0, input.tm_mon);
   ASSERT_EQ(19, input.tm_mday);
-  ASSERT_EQ(5, input.tm_hour);
+  ASSERT_EQ(4, input.tm_hour);
   ASSERT_EQ(14, input.tm_min);
   ASSERT_EQ(7, input.tm_sec);
   ASSERT_EQ(2, input.tm_wday);
diff --git a/libc/test/src/time/localtime_test.cpp b/libc/test/src/time/localtime_test.cpp
index 4143e7065596ff..293f0050c5802b 100644
--- a/libc/test/src/time/localtime_test.cpp
+++ b/libc/test/src/time/localtime_test.cpp
@@ -17,7 +17,7 @@ TEST(LlvmLibcLocaltime, ValidUnixTimestamp0) {
   ASSERT_EQ(70, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
   ASSERT_EQ(1, result->tm_mday);
-  ASSERT_EQ(2, result->tm_hour);
+  ASSERT_EQ(1, result->tm_hour);
   ASSERT_EQ(0, result->tm_min);
   ASSERT_EQ(0, result->tm_sec);
   ASSERT_EQ(4, result->tm_wday);
@@ -31,7 +31,7 @@ TEST(LlvmLibcLocaltime, ValidUnixTimestamp32Int) {
   ASSERT_EQ(138, result->tm_year);
   ASSERT_EQ(0, result->tm_mon);
   ASSERT_EQ(19, result->tm_mday);
-  ASSERT_EQ(5, result->tm_hour);
+  ASSERT_EQ(4, result->tm_hour);
   ASSERT_EQ(14, result->tm_min);
   ASSERT_EQ(7, result->tm_sec);
   ASSERT_EQ(2, result->tm_wday);

>From b3e7a24d06390939220dea8d65e605d4c5881e5d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 21:31:44 +0200
Subject: [PATCH 19/24] update documentation for libc

---
 libc/docs/date_and_time.rst | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/libc/docs/date_and_time.rst b/libc/docs/date_and_time.rst
index ab7c7c3de68677..232faa81155456 100644
--- a/libc/docs/date_and_time.rst
+++ b/libc/docs/date_and_time.rst
@@ -71,9 +71,9 @@ Implementation Status
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
 | clock_settime       |         |         |         |                 |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
-| ctime               |         |         |         |                 |         |         |         |         |         |         |         |         |
+| ctime               | |check| |         |         |                 |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
-| ctime_r             |         |         |         |                 |         |         |         |         |         |         |         |         |
+| ctime_r             | |check| |         |         |                 |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
 | difftime            | |check| | |check| |         |     |check|     |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
@@ -85,11 +85,11 @@ Implementation Status
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
 | gmtime_r            | |check| | |check| |         |     |check|     |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
-| localtime           |         |         |         |                 |         |         |         |         |         |         |         |         |
+| localtime           | |check| |         |         |                 |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
-| localtime_r         |         |         |         |                 |         |         |         |         |         |         |         |         |
+| localtime_r         | |check| |         |         |                 |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
-| localtime_s         |         |         |         |                 |         |         |         |         |         |         |         |         |
+| localtime_s         | |check| |         |         |                 |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+
 | mktime              | |check| | |check| |         |     |check|     |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+

>From f9cb5acdf9eb3f0094089caf40145bf9d3d058bf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 22:31:46 +0200
Subject: [PATCH 20/24] refactor: file read

---
 libc/src/time/time_utils.cpp | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index df55090185f3e2..063d9da4ca408b 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -27,6 +27,26 @@ static int64_t computeRemainingYears(int64_t daysPerYears,
   return years;
 }
 
+volatile int lock = 0;
+
+void release_file(FILE *fp) {
+  lock = 0;
+  fclose(fp);
+}
+
+void acquire_file(FILE *fp, char *timezone) {
+  while (1) {
+    if (lock == 0) {
+      lock = 1;
+      break;
+    }
+  }
+
+  if (fgets(timezone, sizeof(timezone), fp) == NULL) {
+    release_file(fp);
+  }
+}
+
 // First, divide "total_seconds" by the number of seconds in a day to get the
 // number of days since Jan 1 1970. The remainder will be used to calculate the
 // number of Hours, Minutes and Seconds.
@@ -132,13 +152,18 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
 
   char timezone[128];
 
-  FILE *fp;
+  FILE *fp = NULL;
   fp = fopen("/etc/timezone", "rb");
   if (fp == NULL) {
     // TODO: implement getting timezone from `TZ` environment variable and
     // storing the value in `timezone`
-  } else if (fgets(timezone, sizeof(timezone), fp) == NULL)
+  } else {
+    acquire_file(fp, timezone);
+  }
+
+  if (lock == 0) {
     return time_utils::out_of_range();
+  }
 
   // UTC = 0
   int offset = 0;
@@ -168,7 +193,7 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   }
   tm->tm_hour += offset;
 
-  fclose(fp);
+  release_file(fp);
 
   return 0;
 }

>From 4e12c6748e7d0b56fd1bc03f1ee7eb842b30401f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 22:35:51 +0200
Subject: [PATCH 21/24] fix: file read

---
 libc/src/time/time_utils.cpp | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 063d9da4ca408b..54a1b113ddb783 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -27,17 +27,17 @@ static int64_t computeRemainingYears(int64_t daysPerYears,
   return years;
 }
 
-volatile int lock = 0;
+volatile int file_usage = 0;
 
 void release_file(FILE *fp) {
-  lock = 0;
+  file_usage = 0;
   fclose(fp);
 }
 
 void acquire_file(FILE *fp, char *timezone) {
   while (1) {
-    if (lock == 0) {
-      lock = 1;
+    if (file_usage == 0) {
+      file_usage = 1;
       break;
     }
   }
@@ -161,7 +161,8 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
     acquire_file(fp, timezone);
   }
 
-  if (lock == 0) {
+  if (file_usage == 0) {
+    release_file(fp);
     return time_utils::out_of_range();
   }
 

>From ea2120e80ea18e1b1f9c222a22ce8ab78fabf94d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Sun, 6 Oct 2024 22:48:37 +0200
Subject: [PATCH 22/24] fix: timezone char length

---
 libc/src/time/time_utils.cpp | 8 ++++----
 libc/src/time/time_utils.h   | 2 ++
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 54a1b113ddb783..2f33e056b8c1ec 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -34,7 +34,7 @@ void release_file(FILE *fp) {
   fclose(fp);
 }
 
-void acquire_file(FILE *fp, char *timezone) {
+void acquire_file(FILE *fp, char *timezone, size_t timezone_size) {
   while (1) {
     if (file_usage == 0) {
       file_usage = 1;
@@ -42,7 +42,7 @@ void acquire_file(FILE *fp, char *timezone) {
     }
   }
 
-  if (fgets(timezone, sizeof(timezone), fp) == NULL) {
+  if (fgets(timezone, (int)timezone_size, fp) == NULL) {
     release_file(fp);
   }
 }
@@ -150,7 +150,7 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   if (years > INT_MAX || years < INT_MIN)
     return time_utils::out_of_range();
 
-  char timezone[128];
+  char timezone[TimeConstants::TIMEZONE_SIZE];
 
   FILE *fp = NULL;
   fp = fopen("/etc/timezone", "rb");
@@ -158,7 +158,7 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
     // TODO: implement getting timezone from `TZ` environment variable and
     // storing the value in `timezone`
   } else {
-    acquire_file(fp, timezone);
+    acquire_file(fp, timezone, TimeConstants::TIMEZONE_SIZE);
   }
 
   if (file_usage == 0) {
diff --git a/libc/src/time/time_utils.h b/libc/src/time/time_utils.h
index aba8c7f2c14920..780d5242cfa681 100644
--- a/libc/src/time/time_utils.h
+++ b/libc/src/time/time_utils.h
@@ -81,6 +81,8 @@ struct TimeConstants {
   static constexpr int END_OF32_BIT_EPOCH_YEAR = 2038;
 
   static constexpr time_t OUT_OF_RANGE_RETURN_VALUE = -1;
+
+  static constexpr size_t TIMEZONE_SIZE = 128;
 };
 
 // Update the "tm" structure's year, month, etc. members from seconds.

>From 1203b85d1ebc3a495d4ac56fb7e6d479cce13597 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Tue, 8 Oct 2024 15:03:49 +0200
Subject: [PATCH 23/24] timezone implementation

---
 libc/src/time/CMakeLists.txt            |  13 +++
 libc/src/time/time_utils.cpp            |  45 +++++----
 libc/src/time/timezone.cpp              |  52 ++++++++++
 libc/src/time/timezone.h                |  33 +++++++
 libc/test/src/time/CMakeLists.txt       |   5 +
 libc/test/src/time/localtime_r_test.cpp |  24 ++++-
 libc/test/src/time/localtime_test.cpp   | 121 +++++++++++++++++++++++-
 7 files changed, 270 insertions(+), 23 deletions(-)
 create mode 100644 libc/src/time/timezone.cpp
 create mode 100644 libc/src/time/timezone.h

diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
index 57b229d029c92e..008832c5963c25 100644
--- a/libc/src/time/CMakeLists.txt
+++ b/libc/src/time/CMakeLists.txt
@@ -14,6 +14,18 @@ add_object_library(
     libc.src.errno.errno
 )
 
+add_object_library(
+  timezone
+  SRCS
+    timezone.cpp
+  HDRS
+    timezone.h
+  DEPENDS
+    libc.include.time
+    libc.src.__support.CPP.limits
+    libc.src.errno.errno
+)
+
 add_entrypoint_object(
   asctime
   SRCS
@@ -68,6 +80,7 @@ add_entrypoint_object(
     localtime.h
   DEPENDS
     .time_utils
+    .timezone
     libc.hdr.types.time_t
     libc.include.time
 )
diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index 2f33e056b8c1ec..c5d3c88d5c6260 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -7,10 +7,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/time/time_utils.h"
+#include "src/time/timezone.h"
 #include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
+#include "src/__support/CPP/string_view.h"
 #include <stdio.h>
+#include <stdlib.h>
 
 namespace LIBC_NAMESPACE_DECL {
 namespace time_utils {
@@ -29,9 +32,10 @@ static int64_t computeRemainingYears(int64_t daysPerYears,
 
 volatile int file_usage = 0;
 
-void release_file(FILE *fp) {
+void release_file(FILE *fp, char *timezone) {
   file_usage = 0;
   fclose(fp);
+  free(timezone);
 }
 
 void acquire_file(FILE *fp, char *timezone, size_t timezone_size) {
@@ -43,7 +47,7 @@ void acquire_file(FILE *fp, char *timezone, size_t timezone_size) {
   }
 
   if (fgets(timezone, (int)timezone_size, fp) == NULL) {
-    release_file(fp);
+    release_file(fp, timezone);
   }
 }
 
@@ -150,27 +154,25 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   if (years > INT_MAX || years < INT_MIN)
     return time_utils::out_of_range();
 
-  char timezone[TimeConstants::TIMEZONE_SIZE];
-
+  char *timezone = (char *)malloc(sizeof(char) * TimeConstants::TIMEZONE_SIZE);
+  timezone = getenv("TZ");
   FILE *fp = NULL;
-  fp = fopen("/etc/timezone", "rb");
-  if (fp == NULL) {
-    // TODO: implement getting timezone from `TZ` environment variable and
-    // storing the value in `timezone`
-  } else {
+  if (timezone == NULL) {
+    timezone = (char *)realloc(timezone, sizeof(char) * TimeConstants::TIMEZONE_SIZE);
+    fp = fopen("/etc/timezone", "rb");
+    if (fp == NULL) {
+      return time_utils::out_of_range();
+    }
+
     acquire_file(fp, timezone, TimeConstants::TIMEZONE_SIZE);
   }
 
-  if (file_usage == 0) {
-    release_file(fp);
+  if (fp != NULL && file_usage == 0) {
+    release_file(fp, timezone);
     return time_utils::out_of_range();
   }
 
-  // UTC = 0
-  int offset = 0;
-  // TODO: Add more timezones
-  if (internal::same_string(timezone, "Europe/Berlin") == 0)
-    offset = 1;
+  int offset = timezone::get_timezone_offset(timezone);
 
   // All the data (years, month and remaining days) was calculated from
   // March, 2000. Thus adjust the data to be from January, 1900.
@@ -189,12 +191,17 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
       static_cast<int>(remainingSeconds % TimeConstants::SECONDS_PER_MIN);
 
   set_dst(tm);
-  if (tm->tm_isdst > 0) {
+  if (tm->tm_isdst > 0 && offset != 0) {
     tm->tm_hour += 1;
   }
-  tm->tm_hour += offset;
 
-  release_file(fp);
+  if (offset != 0) {
+    tm->tm_hour += offset;
+  }
+
+  if (file_usage == 1) {
+    release_file(fp, timezone);
+  }
 
   return 0;
 }
diff --git a/libc/src/time/timezone.cpp b/libc/src/time/timezone.cpp
new file mode 100644
index 00000000000000..954293ee56bbf0
--- /dev/null
+++ b/libc/src/time/timezone.cpp
@@ -0,0 +1,52 @@
+//===-- Implementation of timezone functions ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/time_utils.h"
+#include "src/time/timezone.h"
+#include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/CPP/string_view.h"
+
+#define BUF_SIZE 1024
+
+namespace LIBC_NAMESPACE_DECL {
+namespace timezone {
+
+using LIBC_NAMESPACE::time_utils::TimeConstants;
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int get_timezone_offset(char *timezone) {
+  int offset = 0;
+  LIBC_NAMESPACE::cpp::string_view tz(timezone);
+
+  if (tz.starts_with("America")) {
+    if (tz.ends_with("San_Francisco")) {
+      offset = -8;
+    }
+
+    if (tz.starts_with("America/New_York")) {
+      offset = -5;
+    }
+  }
+
+  if (tz.starts_with("Europe")) {
+    offset = 1;
+
+    if (tz.ends_with("Moscow")) {
+      offset = 2;
+    }
+  }
+
+  return offset;
+}
+
+} // namespace timezone
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/timezone.h b/libc/src/time/timezone.h
new file mode 100644
index 00000000000000..87f45bdc5d6c2e
--- /dev/null
+++ b/libc/src/time/timezone.h
@@ -0,0 +1,33 @@
+//===-- Implementation of timezone functions ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_TIME_TIMEZONE_H
+#define LLVM_LIBC_SRC_TIME_TIMEZONE_H
+
+#include <stddef.h> // For size_t.
+
+#include "src/__support/CPP/limits.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/CPP/string_view.h"
+#include "src/errno/libc_errno.h"
+#include "src/time/mktime.h"
+
+#include <stdint.h>
+
+namespace LIBC_NAMESPACE_DECL {
+namespace timezone {
+
+#define TZ_HEADER "TZif"
+
+extern int get_timezone_offset(char *timezone);
+
+} // namespace timezone
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_TIME_TIMEZONE_H
diff --git a/libc/test/src/time/CMakeLists.txt b/libc/test/src/time/CMakeLists.txt
index 376bdcac836bab..1c04295613ab4a 100644
--- a/libc/test/src/time/CMakeLists.txt
+++ b/libc/test/src/time/CMakeLists.txt
@@ -46,6 +46,7 @@ add_libc_unittest(
     libc.hdr.types.time_t
     libc.src.time.ctime
     libc.src.time.time_utils
+    libc.src.time.timezone
 )
 
 add_libc_unittest(
@@ -64,6 +65,7 @@ add_libc_unittest(
     libc.hdr.types.time_t
     libc.src.time.ctime_r
     libc.src.time.time_utils
+    libc.src.time.timezone
 )
 
 add_libc_unittest(
@@ -82,6 +84,7 @@ add_libc_unittest(
     libc.hdr.types.time_t
     libc.src.time.localtime
     libc.src.time.time_utils
+    libc.src.time.timezone
 )
 
 add_libc_unittest(
@@ -100,6 +103,7 @@ add_libc_unittest(
     libc.hdr.types.time_t
     libc.src.time.localtime_r
     libc.src.time.time_utils
+    libc.src.time.timezone
 )
 
 add_libc_unittest(
@@ -118,6 +122,7 @@ add_libc_unittest(
     libc.hdr.types.time_t
     libc.src.time.localtime_s
     libc.src.time.time_utils
+    libc.src.time.timezone
 )
 
 add_libc_test(
diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index 03b885fa4aa96b..18b2ef84992226 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -9,11 +9,27 @@
 #include "src/time/localtime_r.h"
 #include "src/time/time_utils.h"
 #include "test/UnitTest/Test.h"
-#include "test/src/time/TmHelper.h"
 
-using LIBC_NAMESPACE::time_utils::TimeConstants;
+#include <string.h>
+
+extern char **environ;
+
+void set_env_var(const char* env) {
+  int i = 0;
+  if (environ[i] != NULL) {
+    i++;
+  }
+
+  environ[i] = (char*)malloc(strlen(env)+1);
+  if (environ[i] != nullptr) {
+    memcpy(environ[i], env, strlen(env)+1);
+    environ[i+1] = nullptr;
+  }
+}
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
+  set_env_var("TZ=Europe/Berlin");
+
   struct tm input;
   time_t t_ptr = 0;
   struct tm *result = LIBC_NAMESPACE::localtime_r(&t_ptr, &input);
@@ -40,6 +56,8 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp0) {
 }
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
+  set_env_var("TZ=Europe/Berlin");
+
   time_t t_ptr = 2147483647;
   struct tm input = (struct tm){.tm_sec = 0,
                                 .tm_min = 0,
@@ -74,6 +92,8 @@ TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32Int) {
 }
 
 TEST(LlvmLibcLocaltimeR, ValidUnixTimestamp32IntDst) {
+  set_env_var("TZ=Europe/Berlin");
+
   time_t t_ptr = 1627225465;
   struct tm input = (struct tm){.tm_sec = 0,
                                 .tm_min = 0,
diff --git a/libc/test/src/time/localtime_test.cpp b/libc/test/src/time/localtime_test.cpp
index 293f0050c5802b..cf373ced8b7fde 100644
--- a/libc/test/src/time/localtime_test.cpp
+++ b/libc/test/src/time/localtime_test.cpp
@@ -6,12 +6,29 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/errno/libc_errno.h"
 #include "src/time/localtime.h"
 #include "test/UnitTest/Test.h"
-#include "test/src/time/TmHelper.h"
+
+#include <string.h>
+
+extern char **environ;
+
+void set_env_var(const char* env) {
+  int i = 0;
+  if (environ[i] != NULL) {
+    i++;
+  }
+
+  environ[i] = (char*)malloc(strlen(env)+1);
+  if (environ[i] != nullptr) {
+    memcpy(environ[i], env, strlen(env)+1);
+    environ[i+1] = nullptr;
+  }
+}
 
 TEST(LlvmLibcLocaltime, ValidUnixTimestamp0) {
+  set_env_var("TZ=Europe/Berlin");
+
   const time_t t_ptr = 0;
   struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
   ASSERT_EQ(70, result->tm_year);
@@ -26,6 +43,8 @@ TEST(LlvmLibcLocaltime, ValidUnixTimestamp0) {
 }
 
 TEST(LlvmLibcLocaltime, ValidUnixTimestamp32Int) {
+  set_env_var("TZ=Europe/Berlin");
+
   time_t t_ptr = 2147483647;
   struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
   ASSERT_EQ(138, result->tm_year);
@@ -40,6 +59,8 @@ TEST(LlvmLibcLocaltime, ValidUnixTimestamp32Int) {
 }
 
 TEST(LlvmLibcLocaltime, ValidUnixTimestamp32IntDst) {
+  set_env_var("TZ=Europe/Berlin");
+
   time_t t_ptr = 1627225465;
   struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
   ASSERT_EQ(121, result->tm_year);
@@ -52,3 +73,99 @@ TEST(LlvmLibcLocaltime, ValidUnixTimestamp32IntDst) {
   ASSERT_EQ(205, result->tm_yday);
   ASSERT_EQ(1, result->tm_isdst);
 }
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestampTzEnvironmentVariableUsaPst) {
+  set_env_var("TZ=America/San_Francisco");
+
+  time_t t_ptr = 1627225465;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(8, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestampTzEnvironmentVariableUsaEst) {
+  set_env_var("TZ=America/New_York");
+
+  time_t t_ptr = 1627225465;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(11, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestampTzEnvironmentVariableUTC) {
+  set_env_var("TZ=UTC");
+
+  time_t t_ptr = 1627225465;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(15, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestampTzEnvironmentVariableGMT) {
+  set_env_var("TZ=GMT");
+
+  time_t t_ptr = 1627225465;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(15, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestampTzEnvironmentVariableEuropeBerlin) {
+  set_env_var("TZ=Europe/Berlin");
+
+  time_t t_ptr = 1627225465;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(17, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}
+
+TEST(LlvmLibcLocaltime, ValidUnixTimestampTzEnvironmentVariableRussiaMoscow) {
+  set_env_var("TZ=Europe/Moscow");
+
+  time_t t_ptr = 1627225465;
+  struct tm *result = LIBC_NAMESPACE::localtime(&t_ptr);
+  ASSERT_EQ(121, result->tm_year);
+  ASSERT_EQ(6, result->tm_mon);
+  ASSERT_EQ(25, result->tm_mday);
+  ASSERT_EQ(18, result->tm_hour);
+  ASSERT_EQ(4, result->tm_min);
+  ASSERT_EQ(25, result->tm_sec);
+  ASSERT_EQ(0, result->tm_wday);
+  ASSERT_EQ(205, result->tm_yday);
+  ASSERT_EQ(1, result->tm_isdst);
+}

>From 469939ea800b9523c22cb0eeef3f9627445a069e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=97=D0=B8=D1=88=D0=B0=D0=BD=20=D0=9C=D0=B8=D1=80=D0=B7?=
 =?UTF-8?q?=D0=B0?= <zmirza at tutanota.de>
Date: Tue, 8 Oct 2024 15:04:22 +0200
Subject: [PATCH 24/24] format code with clang-format

---
 libc/src/time/time_utils.cpp            | 7 ++++---
 libc/src/time/timezone.cpp              | 4 ++--
 libc/src/time/timezone.h                | 2 +-
 libc/test/src/time/localtime_r_test.cpp | 8 ++++----
 libc/test/src/time/localtime_test.cpp   | 8 ++++----
 5 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/libc/src/time/time_utils.cpp b/libc/src/time/time_utils.cpp
index c5d3c88d5c6260..31034a8fb66428 100644
--- a/libc/src/time/time_utils.cpp
+++ b/libc/src/time/time_utils.cpp
@@ -7,11 +7,11 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/time/time_utils.h"
-#include "src/time/timezone.h"
 #include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
+#include "src/__support/CPP/string_view.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
-#include "src/__support/CPP/string_view.h"
+#include "src/time/timezone.h"
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -158,7 +158,8 @@ int64_t update_from_seconds(int64_t total_seconds, struct tm *tm) {
   timezone = getenv("TZ");
   FILE *fp = NULL;
   if (timezone == NULL) {
-    timezone = (char *)realloc(timezone, sizeof(char) * TimeConstants::TIMEZONE_SIZE);
+    timezone =
+        (char *)realloc(timezone, sizeof(char) * TimeConstants::TIMEZONE_SIZE);
     fp = fopen("/etc/timezone", "rb");
     if (fp == NULL) {
       return time_utils::out_of_range();
diff --git a/libc/src/time/timezone.cpp b/libc/src/time/timezone.cpp
index 954293ee56bbf0..8fd3cb86425fac 100644
--- a/libc/src/time/timezone.cpp
+++ b/libc/src/time/timezone.cpp
@@ -6,12 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/time/time_utils.h"
 #include "src/time/timezone.h"
 #include "src/__support/CPP/limits.h" // INT_MIN, INT_MAX
+#include "src/__support/CPP/string_view.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
-#include "src/__support/CPP/string_view.h"
+#include "src/time/time_utils.h"
 
 #define BUF_SIZE 1024
 
diff --git a/libc/src/time/timezone.h b/libc/src/time/timezone.h
index 87f45bdc5d6c2e..26bf52c6cb9286 100644
--- a/libc/src/time/timezone.h
+++ b/libc/src/time/timezone.h
@@ -12,9 +12,9 @@
 #include <stddef.h> // For size_t.
 
 #include "src/__support/CPP/limits.h"
+#include "src/__support/CPP/string_view.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
-#include "src/__support/CPP/string_view.h"
 #include "src/errno/libc_errno.h"
 #include "src/time/mktime.h"
 
diff --git a/libc/test/src/time/localtime_r_test.cpp b/libc/test/src/time/localtime_r_test.cpp
index 18b2ef84992226..ca9b2014dbe785 100644
--- a/libc/test/src/time/localtime_r_test.cpp
+++ b/libc/test/src/time/localtime_r_test.cpp
@@ -14,16 +14,16 @@
 
 extern char **environ;
 
-void set_env_var(const char* env) {
+void set_env_var(const char *env) {
   int i = 0;
   if (environ[i] != NULL) {
     i++;
   }
 
-  environ[i] = (char*)malloc(strlen(env)+1);
+  environ[i] = (char *)malloc(strlen(env) + 1);
   if (environ[i] != nullptr) {
-    memcpy(environ[i], env, strlen(env)+1);
-    environ[i+1] = nullptr;
+    memcpy(environ[i], env, strlen(env) + 1);
+    environ[i + 1] = nullptr;
   }
 }
 
diff --git a/libc/test/src/time/localtime_test.cpp b/libc/test/src/time/localtime_test.cpp
index cf373ced8b7fde..2689a1397cb068 100644
--- a/libc/test/src/time/localtime_test.cpp
+++ b/libc/test/src/time/localtime_test.cpp
@@ -13,16 +13,16 @@
 
 extern char **environ;
 
-void set_env_var(const char* env) {
+void set_env_var(const char *env) {
   int i = 0;
   if (environ[i] != NULL) {
     i++;
   }
 
-  environ[i] = (char*)malloc(strlen(env)+1);
+  environ[i] = (char *)malloc(strlen(env) + 1);
   if (environ[i] != nullptr) {
-    memcpy(environ[i], env, strlen(env)+1);
-    environ[i+1] = nullptr;
+    memcpy(environ[i], env, strlen(env) + 1);
+    environ[i + 1] = nullptr;
   }
 }
 



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