[libcxx-commits] [libcxx] e5d2d3e - [libc++][chrono] Implements formatter day.

Mark de Wever via libcxx-commits libcxx-commits at lists.llvm.org
Wed Sep 7 09:44:13 PDT 2022


Author: Mark de Wever
Date: 2022-09-07T18:44:04+02:00
New Revision: e5d2d3eafbb367450c4fb75d3ee72b3b4bba522b

URL: https://github.com/llvm/llvm-project/commit/e5d2d3eafbb367450c4fb75d3ee72b3b4bba522b
DIFF: https://github.com/llvm/llvm-project/commit/e5d2d3eafbb367450c4fb75d3ee72b3b4bba522b.diff

LOG: [libc++][chrono] Implements formatter day.

This implements the enabled specializaton
template<class charT> struct formatter<chrono::day, charT>;

and
template<class charT, class traits>
    basic_ostream<charT, traits>&
      operator<<(basic_ostream<charT, traits>& os, const day& d);

Implements:
- LWG 3241 chrono-spec grammar ambiguity in §[time.format]

Partially implements:
- P1361 Integration of chrono with text formatting

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D128577

Added: 
    libcxx/include/__chrono/convert_to_tm.h
    libcxx/include/__chrono/formatter.h
    libcxx/include/__chrono/ostream.h
    libcxx/include/__chrono/parser_std_format_spec.h
    libcxx/include/__chrono/statically_widen.h
    libcxx/test/std/time/time.cal/time.cal.day/time.cal.day.nonmembers/ostream.pass.cpp
    libcxx/test/std/time/time.syn/formatter.day.pass.cpp
    libcxx/test/std/time/time.syn/formatter_tests.h

Modified: 
    libcxx/docs/Status/Cxx20Issues.csv
    libcxx/docs/Status/Cxx20Papers.csv
    libcxx/include/CMakeLists.txt
    libcxx/include/__format/parser_std_format_spec.h
    libcxx/include/chrono
    libcxx/include/module.modulemap.in
    libcxx/test/libcxx/private_headers.verify.cpp
    libcxx/test/libcxx/transitive_includes/cxx20/expected.chrono
    libcxx/test/libcxx/transitive_includes/cxx2b/expected.chrono
    libcxx/test/libcxx/utilities/format/format.formatter/format.formatter.spec/formattable.compile.pass.cpp
    libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/types.compile.pass.cpp
    libcxx/utils/ci/run-buildbot

Removed: 
    


################################################################################
diff  --git a/libcxx/docs/Status/Cxx20Issues.csv b/libcxx/docs/Status/Cxx20Issues.csv
index 1cdbeac9a4634..4525a53999eda 100644
--- a/libcxx/docs/Status/Cxx20Issues.csv
+++ b/libcxx/docs/Status/Cxx20Issues.csv
@@ -175,7 +175,7 @@
 "`3253 <https://wg21.link/LWG3253>`__","``basic_syncbuf::basic_syncbuf()``\  should not be explicit","Belfast","",""
 "`3245 <https://wg21.link/LWG3245>`__","Unnecessary restriction on ``'%p'``\  parse specifier","Belfast","","","|chrono|"
 "`3244 <https://wg21.link/LWG3244>`__","Constraints for ``Source``\  in |sect|\ [fs.path.req] insufficiently constrainty","Belfast","",""
-"`3241 <https://wg21.link/LWG3241>`__","``chrono-spec``\  grammar ambiguity in |sect|\ [time.format]","Belfast","","","|chrono| |format|"
+"`3241 <https://wg21.link/LWG3241>`__","``chrono-spec``\  grammar ambiguity in |sect|\ [time.format]","Belfast","|Complete|","16.0","|chrono| |format|"
 "`3257 <https://wg21.link/LWG3257>`__","Missing feature testing macro update from P0858","Belfast","",""
 "`3256 <https://wg21.link/LWG3256>`__","Feature testing macro for ``constexpr``\  algorithms","Belfast","|Complete|","13.0"
 "`3273 <https://wg21.link/LWG3273>`__","Specify ``weekday_indexed``\  to range of ``[0, 7]``\ ","Belfast","","","|chrono|"

diff  --git a/libcxx/docs/Status/Cxx20Papers.csv b/libcxx/docs/Status/Cxx20Papers.csv
index a3b044080da26..f9c58b2be7384 100644
--- a/libcxx/docs/Status/Cxx20Papers.csv
+++ b/libcxx/docs/Status/Cxx20Papers.csv
@@ -114,7 +114,7 @@
 "`P1207R4 <https://wg21.link/P1207R4>`__","LWG","Movability of Single-pass Iterators","Cologne","|Complete|","15.0"
 "`P1208R6 <https://wg21.link/P1208R6>`__","LWG","Adopt source_location for C++20","Cologne","",""
 "`P1355R2 <https://wg21.link/P1355R2>`__","LWG","Exposing a narrow contract for ceil2","Cologne","|Complete|","9.0"
-"`P1361R2 <https://wg21.link/P1361R2>`__","LWG","Integration of chrono with text formatting","Cologne","",""
+"`P1361R2 <https://wg21.link/P1361R2>`__","LWG","Integration of chrono with text formatting","Cologne","|In Progress|",""
 "`P1423R3 <https://wg21.link/P1423R3>`__","LWG","char8_t backward compatibility remediation","Cologne","|Complete|","15.0"
 "`P1424R1 <https://wg21.link/P1424R1>`__","LWG","'constexpr' feature macro concerns","Cologne","Superseded by `P1902 <https://wg21.link/P1902>`__",""
 "`P1466R3 <https://wg21.link/P1466R3>`__","LWG","Miscellaneous minor fixes for chrono","Cologne","",""

diff  --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index cce8db80a6c6c..080d9ea8a5d1f 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -204,15 +204,20 @@ set(files
   __charconv/to_chars_result.h
   __chrono/calendar.h
   __chrono/convert_to_timespec.h
+  __chrono/convert_to_tm.h
   __chrono/day.h
   __chrono/duration.h
   __chrono/file_clock.h
+  __chrono/formatter.h
   __chrono/hh_mm_ss.h
   __chrono/high_resolution_clock.h
   __chrono/literals.h
   __chrono/month.h
   __chrono/month_weekday.h
   __chrono/monthday.h
+  __chrono/ostream.h
+  __chrono/parser_std_format_spec.h
+  __chrono/statically_widen.h
   __chrono/steady_clock.h
   __chrono/system_clock.h
   __chrono/time_point.h

diff  --git a/libcxx/include/__chrono/convert_to_tm.h b/libcxx/include/__chrono/convert_to_tm.h
new file mode 100644
index 0000000000000..a5dc3beaa2c0a
--- /dev/null
+++ b/libcxx/include/__chrono/convert_to_tm.h
@@ -0,0 +1,45 @@
+// -*- 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 _LIBCPP___CHRONO_CONVERT_TO_TM_H
+#define _LIBCPP___CHRONO_CONVERT_TO_TM_H
+
+#include <__chrono/day.h>
+#include <__concepts/same_as.h>
+#include <__config>
+#include <ctime>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI tm __convert_to_tm(const _Tp& __value) {
+  tm __result = {};
+#  ifdef __GLIBC__
+  __result.tm_zone = "UTC";
+#  endif
+
+  if constexpr (same_as<_Tp, chrono::day>)
+    __result.tm_mday = static_cast<unsigned>(__value);
+  else
+    static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
+
+  return __result;
+}
+
+#endif //if _LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CHRONO_CONVERT_TO_TM_H

diff  --git a/libcxx/include/__chrono/formatter.h b/libcxx/include/__chrono/formatter.h
new file mode 100644
index 0000000000000..a38a5538228a2
--- /dev/null
+++ b/libcxx/include/__chrono/formatter.h
@@ -0,0 +1,153 @@
+// -*- 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 _LIBCPP___CHRONO_FORMATTER_H
+#define _LIBCPP___CHRONO_FORMATTER_H
+
+#include <__chrono/convert_to_tm.h>
+#include <__chrono/day.h>
+#include <__chrono/parser_std_format_spec.h>
+#include <__config>
+#include <__format/concepts.h>
+#include <__format/format_parse_context.h>
+#include <__format/formatter.h>
+#include <__format/formatter_output.h>
+#include <__format/parser_std_format_spec.h>
+#include <ctime>
+#include <sstream>
+#include <string>
+#include <string_view>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
+
+namespace __formatter {
+
+/// Formats a time based on a tm struct.
+///
+/// This formatter passes the formatting to time_put which uses strftime. When
+/// the value is outside the valid range it's unspecified what strftime will
+/// output. For example weekday 8 can print 1 when the day is processed modulo
+/// 7 since that handles the Sunday for 0-based weekday. It can also print 8 if
+/// 7 is handled as a special case.
+///
+/// The Standard doesn't specify what to do in this case so the result depends
+/// on the result of the underlying code.
+///
+/// \pre When the (abbreviated) weekday or month name are used, the caller
+///      validates whether the value is valid. So the caller handles that
+///      requirement of Table 97: Meaning of conversion specifiers
+///      [tab:time.format.spec].
+///
+/// When no chrono-specs are provided it uses the stream formatter.
+
+template <class _CharT, class _Tp>
+_LIBCPP_HIDE_FROM_ABI void __format_chrono_using_chrono_specs(
+    const _Tp& __value, basic_stringstream<_CharT>& __sstr, basic_string_view<_CharT> __chrono_specs) {
+  tm __t              = std::__convert_to_tm(__value);
+  const auto& __facet = std::use_facet<time_put<_CharT>>(__sstr.getloc());
+  for (auto __it = __chrono_specs.begin(); __it != __chrono_specs.end(); ++__it) {
+    if (*__it == _CharT('%')) {
+      auto __s = __it;
+      ++__it;
+      // We only handle the types that can't be directly handled by time_put.
+      // (as an optimization n, t, and % are also handled directly.)
+      switch (*__it) {
+      case _CharT('n'):
+        __sstr << _CharT('\n');
+        break;
+      case _CharT('t'):
+        __sstr << _CharT('\t');
+        break;
+      case _CharT('%'):
+        __sstr << *__it;
+        break;
+
+      case _CharT('O'):
+        ++__it;
+        [[fallthrough]];
+      default:
+        __facet.put({__sstr}, __sstr, _CharT(' '), std::addressof(__t), __s, __it + 1);
+        break;
+      }
+    } else {
+      __sstr << *__it;
+    }
+  }
+}
+
+template <class _CharT, class _Tp>
+_LIBCPP_HIDE_FROM_ABI auto
+__format_chrono(const _Tp& __value,
+                auto& __ctx,
+                __format_spec::__parsed_specifications<_CharT> __specs,
+                basic_string_view<_CharT> __chrono_specs) -> decltype(__ctx.out()) {
+  basic_stringstream<_CharT> __sstr;
+  // [time.format]/2
+  // 2.1 - the "C" locale if the L option is not present in chrono-format-spec, otherwise
+  // 2.2 - the locale passed to the formatting function if any, otherwise
+  // 2.3 - the global locale.
+  // Note that the __ctx's locale() call does 2.2 and 2.3.
+  if (__specs.__chrono_.__locale_specific_form_)
+    __sstr.imbue(__ctx.locale());
+  else
+    __sstr.imbue(locale::classic());
+
+  if (__chrono_specs.empty())
+    __sstr << __value;
+  else
+    __formatter::__format_chrono_using_chrono_specs(__value, __sstr, __chrono_specs);
+
+  // TODO FMT Use the stringstream's view after P0408R7 has been implemented.
+  basic_string<_CharT> __str = __sstr.str();
+  return __formatter::__write_string(basic_string_view<_CharT>{__str}, __ctx.out(), __specs);
+}
+
+} // namespace __formatter
+
+template <__fmt_char_type _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT __formatter_chrono {
+public:
+  _LIBCPP_HIDE_FROM_ABI constexpr auto __parse(
+      basic_format_parse_context<_CharT>& __parse_ctx, __format_spec::__fields __fields, __format_spec::__flags __flags)
+      -> decltype(__parse_ctx.begin()) {
+    return __parser_.__parse(__parse_ctx, __fields, __flags);
+  }
+
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI auto format(const _Tp& __value, auto& __ctx) const -> decltype(__ctx.out()) const {
+    return __formatter::__format_chrono(
+        __value, __ctx, __parser_.__parser_.__get_parsed_chrono_specifications(__ctx), __parser_.__chrono_specs_);
+  }
+
+  __format_spec::__parser_chrono<_CharT> __parser_;
+};
+
+template <__fmt_char_type _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<chrono::day, _CharT>
+    : public __formatter_chrono<_CharT> {
+public:
+  using _Base = __formatter_chrono<_CharT>;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto parse(basic_format_parse_context<_CharT>& __parse_ctx)
+      -> decltype(__parse_ctx.begin()) {
+    return _Base::__parse(__parse_ctx, __format_spec::__fields_chrono, __format_spec::__flags::__day);
+  }
+};
+
+#endif //if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif //  _LIBCPP___CHRONO_FORMATTER_H

diff  --git a/libcxx/include/__chrono/ostream.h b/libcxx/include/__chrono/ostream.h
new file mode 100644
index 0000000000000..f45e949b67ba6
--- /dev/null
+++ b/libcxx/include/__chrono/ostream.h
@@ -0,0 +1,49 @@
+// -*- 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 _LIBCPP___CHRONO_OSTREAM_H
+#define _LIBCPP___CHRONO_OSTREAM_H
+
+#include <__chrono/day.h>
+#include <__chrono/statically_widen.h>
+#include <__config>
+#include <format>
+#include <ostream>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
+
+namespace chrono {
+
+template <class _CharT, class _Traits>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, const day& __d) {
+  return __os
+      << (__d.ok()
+              ? std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:%d}"), __d)
+              // Note this error 
diff ers from the wording of the Standard. The
+              // Standard wording doesn't work well on AIX or Windows. There
+              // the formatted day seems to be either modulo 100 or completely
+              // omitted. Judging by the wording this is valid.
+              // TODO FMT Write a paper of file an LWG issue.
+              : std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:02} is not a valid day"), static_cast<unsigned>(__d)));
+}
+
+} // namespace chrono
+
+#endif //if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CHRONO_OSTREAM_H

diff  --git a/libcxx/include/__chrono/parser_std_format_spec.h b/libcxx/include/__chrono/parser_std_format_spec.h
new file mode 100644
index 0000000000000..3734cddceee87
--- /dev/null
+++ b/libcxx/include/__chrono/parser_std_format_spec.h
@@ -0,0 +1,401 @@
+// -*- 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 _LIBCPP___CHRONO_PARSER_STD_FORMAT_SPEC_H
+#define _LIBCPP___CHRONO_PARSER_STD_FORMAT_SPEC_H
+
+#include <__config>
+#include <__format/concepts.h>
+#include <__format/format_error.h>
+#include <__format/format_parse_context.h>
+#include <__format/formatter_string.h>
+#include <__format/parser_std_format_spec.h>
+#include <string_view>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
+
+namespace __format_spec {
+
+// By not placing this constant in the formatter class it's not duplicated for char and wchar_t
+inline constexpr __fields __fields_chrono{.__locale_specific_form_ = true, .__type_ = false};
+
+/// Flags available or required in a chrono type.
+///
+/// The caller of the chrono formatter lists the types it has available and the
+/// validation tests whether the requested type spec (e.g. %M) is available in
+/// the formatter.
+/// When the type in the chrono-format-spec isn't present in the data a
+/// \ref format_error is thrown.
+enum class __flags {
+  __second = 0x1,
+  __minute = 0x2,
+  __hour   = 0x4,
+  __time   = __hour | __minute | __second,
+
+  __day   = 0x8,
+  __month = 0x10,
+  __year  = 0x20,
+
+  __weekday = 0x40,
+
+  __month_day     = __day | __month,
+  __month_weekday = __weekday | __month,
+  __year_month    = __month | __year,
+  __date          = __day | __month | __year | __weekday,
+
+  __date_time = __date | __time,
+
+  __duration = 0x80 | __time,
+
+  __time_zone = 0x100,
+
+  __clock = __date_time | __time_zone
+};
+
+_LIBCPP_HIDE_FROM_ABI constexpr __flags operator&(__flags __lhs, __flags __rhs) {
+  return static_cast<__flags>(static_cast<unsigned>(__lhs) & static_cast<unsigned>(__rhs));
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_second(__flags __flags) {
+  if ((__flags & __flags::__second) != __flags::__second)
+    std::__throw_format_error("The supplied date time doesn't contain a second");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_minute(__flags __flags) {
+  if ((__flags & __flags::__minute) != __flags::__minute)
+    std::__throw_format_error("The supplied date time doesn't contain a minute");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_hour(__flags __flags) {
+  if ((__flags & __flags::__hour) != __flags::__hour)
+    std::__throw_format_error("The supplied date time doesn't contain an hour");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_time(__flags __flags) {
+  if ((__flags & __flags::__time) != __flags::__time)
+    std::__throw_format_error("The supplied date time doesn't contain a time");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_day(__flags __flags) {
+  if ((__flags & __flags::__day) != __flags::__day)
+    std::__throw_format_error("The supplied date time doesn't contain a day");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_month(__flags __flags) {
+  if ((__flags & __flags::__month) != __flags::__month)
+    std::__throw_format_error("The supplied date time doesn't contain a month");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_year(__flags __flags) {
+  if ((__flags & __flags::__year) != __flags::__year)
+    std::__throw_format_error("The supplied date time doesn't contain a year");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_date(__flags __flags) {
+  if ((__flags & __flags::__date) != __flags::__date)
+    std::__throw_format_error("The supplied date time doesn't contain a date");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_date_or_duration(__flags __flags) {
+  if (((__flags & __flags::__date) != __flags::__date) && ((__flags & __flags::__duration) != __flags::__duration))
+    std::__throw_format_error("The supplied date time doesn't contain a date or duration");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_date_time(__flags __flags) {
+  if ((__flags & __flags::__date_time) != __flags::__date_time)
+    std::__throw_format_error("The supplied date time doesn't contain a date and time");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_weekday(__flags __flags) {
+  if ((__flags & __flags::__weekday) != __flags::__weekday)
+    std::__throw_format_error("The supplied date time doesn't contain a weekday");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_duration(__flags __flags) {
+  if ((__flags & __flags::__duration) != __flags::__duration)
+    std::__throw_format_error("The supplied date time doesn't contain a duration");
+}
+
+_LIBCPP_HIDE_FROM_ABI constexpr void __validate_time_zone(__flags __flags) {
+  if ((__flags & __flags::__time_zone) != __flags::__time_zone)
+    std::__throw_format_error("The supplied date time doesn't contain a time zone");
+}
+
+template <class _CharT>
+class _LIBCPP_TEMPLATE_VIS __parser_chrono {
+public:
+  _LIBCPP_HIDE_FROM_ABI constexpr auto
+  __parse(basic_format_parse_context<_CharT>& __parse_ctx, __fields __fields, __flags __flags)
+      -> decltype(__parse_ctx.begin()) {
+    const _CharT* __begin = __parser_.__parse(__parse_ctx, __fields);
+    const _CharT* __end   = __parse_ctx.end();
+    if (__begin == __end)
+      return __begin;
+
+    const _CharT* __last = __parse_chrono_specs(__begin, __end, __flags);
+    __chrono_specs_      = basic_string_view<_CharT>{__begin, __last};
+
+    return __last;
+  }
+
+  __parser<_CharT> __parser_;
+  basic_string_view<_CharT> __chrono_specs_;
+
+private:
+  _LIBCPP_HIDE_FROM_ABI constexpr const _CharT*
+  __parse_chrono_specs(const _CharT* __begin, const _CharT* __end, __flags __flags) {
+    _LIBCPP_ASSERT(__begin != __end,
+                   "When called with an empty input the function will cause "
+                   "undefined behavior by evaluating data not in the input");
+
+    if (*__begin != _CharT('%') && *__begin != _CharT('}'))
+      std::__throw_format_error("Expected '%' or '}' in the chrono format-string");
+
+    do {
+      switch (*__begin) {
+      case _CharT('{'):
+        std::__throw_format_error("The chrono-specs contains a '{'");
+
+      case _CharT('}'):
+        return __begin;
+
+      case _CharT('%'):
+        __parse_conversion_spec(__begin, __end, __flags);
+        [[fallthrough]];
+
+      default:
+        // All other literals
+        ++__begin;
+      }
+
+    } while (__begin != __end && *__begin != _CharT('}'));
+
+    return __begin;
+  }
+
+  /// \pre *__begin == '%'
+  /// \post __begin points at the end parsed conversion-spec
+  _LIBCPP_HIDE_FROM_ABI constexpr void
+  __parse_conversion_spec(const _CharT*& __begin, const _CharT* __end, __flags __flags) {
+    ++__begin;
+    if (__begin == __end)
+      std::__throw_format_error("End of input while parsing the modifier chrono conversion-spec");
+
+    switch (*__begin) {
+    case _CharT('n'):
+    case _CharT('t'):
+    case _CharT('%'):
+      break;
+
+    case _CharT('S'):
+      __format_spec::__validate_second(__flags);
+      break;
+
+    case _CharT('M'):
+      __format_spec::__validate_minute(__flags);
+      break;
+
+    case _CharT('p'): // TODO FMT does the formater require an hour or a time?
+    case _CharT('H'):
+    case _CharT('I'):
+      __validate_hour(__flags);
+      break;
+
+    case _CharT('r'):
+    case _CharT('R'):
+    case _CharT('T'):
+    case _CharT('X'):
+      __format_spec::__validate_time(__flags);
+      break;
+
+    case _CharT('d'):
+    case _CharT('e'):
+      __format_spec::__validate_day(__flags);
+      break;
+
+    case _CharT('b'):
+    case _CharT('h'):
+    case _CharT('B'):
+      __parser_.__month_name_ = true;
+      [[fallthrough]];
+    case _CharT('m'):
+      __format_spec::__validate_month(__flags);
+      break;
+
+    case _CharT('y'):
+    case _CharT('C'):
+    case _CharT('Y'):
+      __format_spec::__validate_year(__flags);
+      break;
+
+    case _CharT('j'):
+      __format_spec::__validate_date_or_duration(__flags);
+      break;
+
+    case _CharT('g'):
+    case _CharT('x'):
+    case _CharT('D'):
+    case _CharT('F'):
+    case _CharT('G'):
+    case _CharT('U'):
+    case _CharT('V'):
+    case _CharT('W'):
+      __format_spec::__validate_date(__flags);
+      break;
+
+    case _CharT('c'):
+      __format_spec::__validate_date_time(__flags);
+      break;
+
+    case _CharT('a'):
+    case _CharT('A'):
+      __parser_.__weekday_name_ = true;
+      [[fallthrough]];
+    case _CharT('u'):
+    case _CharT('w'):
+      __format_spec::__validate_weekday(__flags);
+      break;
+
+    case _CharT('q'):
+    case _CharT('Q'):
+      __format_spec::__validate_duration(__flags);
+      break;
+
+    case _CharT('E'):
+      __parse_modifier_E(__begin, __end, __flags);
+      break;
+
+    case _CharT('O'):
+      __parse_modifier_O(__begin, __end, __flags);
+      break;
+
+    case _CharT('z'):
+    case _CharT('Z'):
+      // Currently there's no time zone information. However some clocks have a
+      // hard-coded "time zone", for these clocks the information can be used.
+      // TODO FMT implement time zones.
+      __format_spec::__validate_time_zone(__flags);
+      break;
+
+    default: // unknown type;
+      std::__throw_format_error("The date time type specifier is invalid");
+    }
+  }
+
+  /// \pre *__begin == 'E'
+  /// \post __begin is incremented by one.
+  _LIBCPP_HIDE_FROM_ABI constexpr void
+  __parse_modifier_E(const _CharT*& __begin, const _CharT* __end, __flags __flags) {
+    ++__begin;
+    if (__begin == __end)
+      std::__throw_format_error("End of input while parsing the modifier E");
+
+    switch (*__begin) {
+    case _CharT('X'):
+      __format_spec::__validate_time(__flags);
+      break;
+
+    case _CharT('y'):
+    case _CharT('C'):
+    case _CharT('Y'):
+      __format_spec::__validate_year(__flags);
+      break;
+
+    case _CharT('x'):
+      __format_spec::__validate_date(__flags);
+      break;
+
+    case _CharT('c'):
+      __format_spec::__validate_date_time(__flags);
+      break;
+
+    case _CharT('z'):
+      // Currently there's no time zone information. However some clocks have a
+      // hard-coded "time zone", for these clocks the information can be used.
+      // TODO FMT implement time zones.
+      __format_spec::__validate_time_zone(__flags);
+      break;
+
+    default:
+      std::__throw_format_error("The date time type specifier for modifier E is invalid");
+    }
+  }
+
+  /// \pre *__begin == 'O'
+  /// \post __begin is incremented by one.
+  _LIBCPP_HIDE_FROM_ABI constexpr void
+  __parse_modifier_O(const _CharT*& __begin, const _CharT* __end, __flags __flags) {
+    ++__begin;
+    if (__begin == __end)
+      std::__throw_format_error("End of input while parsing the modifier O");
+
+    switch (*__begin) {
+    case _CharT('S'):
+      __format_spec::__validate_second(__flags);
+      break;
+
+    case _CharT('M'):
+      __format_spec::__validate_minute(__flags);
+      break;
+
+    case _CharT('I'):
+    case _CharT('H'):
+      __format_spec::__validate_hour(__flags);
+      break;
+
+    case _CharT('d'):
+    case _CharT('e'):
+      __format_spec::__validate_day(__flags);
+      break;
+
+    case _CharT('m'):
+      __format_spec::__validate_month(__flags);
+      break;
+
+    case _CharT('y'):
+      __format_spec::__validate_year(__flags);
+      break;
+
+    case _CharT('U'):
+    case _CharT('V'):
+    case _CharT('W'):
+      __format_spec::__validate_date(__flags);
+      break;
+
+    case _CharT('u'):
+    case _CharT('w'):
+      __format_spec::__validate_weekday(__flags);
+      break;
+
+    case _CharT('z'):
+      // Currently there's no time zone information. However some clocks have a
+      // hard-coded "time zone", for these clocks the information can be used.
+      // TODO FMT implement time zones.
+      __format_spec::__validate_time_zone(__flags);
+      break;
+
+    default:
+      std::__throw_format_error("The date time type specifier for modifier O is invalid");
+    }
+  }
+};
+
+} // namespace __format_spec
+
+#endif //_LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CHRONO_PARSER_STD_FORMAT_SPEC_H

diff  --git a/libcxx/include/__chrono/statically_widen.h b/libcxx/include/__chrono/statically_widen.h
new file mode 100644
index 0000000000000..dd12c3f5020e3
--- /dev/null
+++ b/libcxx/include/__chrono/statically_widen.h
@@ -0,0 +1,52 @@
+// -*- 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 _LIBCPP___CHRONO_STATICALLY_WIDEN_H
+#define _LIBCPP___CHRONO_STATICALLY_WIDEN_H
+
+// Implements the STATICALLY-WIDEN exposition-only function. ([time.general]/2)
+
+#include <__concepts/same_as.h>
+#include <__config>
+#include <__format/concepts.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+#  ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+template <__fmt_char_type _CharT>
+consteval const _CharT* __statically_widen(const char* __str, const wchar_t* __wstr) {
+  if constexpr (same_as<_CharT, char>)
+    return __str;
+  else
+    return __wstr;
+}
+#    define _LIBCPP_STATICALLY_WIDEN(_CharT, __str) ::std::__statically_widen<_CharT>(__str, L##__str)
+#  else // _LIBCPP_HAS_NO_WIDE_CHARACTERS
+
+// Without this indirection the unit test test/libcxx/modules_include.sh.cpp
+// fails for the CI build "No wide characters". This seems like a bug.
+// TODO FMT investigate why this is needed.
+template <__fmt_char_type _CharT>
+consteval const _CharT* __statically_widen(const char* __str) {
+  return __str;
+}
+#    define _LIBCPP_STATICALLY_WIDEN(_CharT, __str) ::std::__statically_widen<_CharT>(__str)
+#  endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
+
+#endif //if _LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___CHRONO_STATICALLY_WIDEN_H

diff  --git a/libcxx/include/__format/parser_std_format_spec.h b/libcxx/include/__format/parser_std_format_spec.h
index 576cded812bbc..128f1d9c38d5c 100644
--- a/libcxx/include/__format/parser_std_format_spec.h
+++ b/libcxx/include/__format/parser_std_format_spec.h
@@ -176,6 +176,7 @@ struct __std {
 
 struct __chrono {
   __alignment __alignment_ : 3;
+  bool __locale_specific_form_ : 1;
   bool __weekday_name_ : 1;
   bool __month_name_ : 1;
 };
@@ -288,12 +289,22 @@ class _LIBCPP_TEMPLATE_VIS __parser {
   _LIBCPP_HIDE_FROM_ABI
   __parsed_specifications<_CharT> __get_parsed_std_specifications(auto& __ctx) const {
     return __parsed_specifications<_CharT>{
-        .__std_ =
-            __std{.__alignment_            = __alignment_,
-                  .__sign_                 = __sign_,
-                  .__alternate_form_       = __alternate_form_,
-                  .__locale_specific_form_ = __locale_specific_form_,
-                  .__type_                 = __type_},
+        .__std_ = __std{.__alignment_            = __alignment_,
+                        .__sign_                 = __sign_,
+                        .__alternate_form_       = __alternate_form_,
+                        .__locale_specific_form_ = __locale_specific_form_,
+                        .__type_                 = __type_},
+        .__width_{__get_width(__ctx)},
+        .__precision_{__get_precision(__ctx)},
+        .__fill_{__fill_}};
+  }
+
+  _LIBCPP_HIDE_FROM_ABI __parsed_specifications<_CharT> __get_parsed_chrono_specifications(auto& __ctx) const {
+    return __parsed_specifications<_CharT>{
+        .__chrono_ = __chrono{.__alignment_            = __alignment_,
+                              .__locale_specific_form_ = __locale_specific_form_,
+                              .__weekday_name_         = __weekday_name_,
+                              .__month_name_           = __month_name_},
         .__width_{__get_width(__ctx)},
         .__precision_{__get_precision(__ctx)},
         .__fill_{__fill_}};

diff  --git a/libcxx/include/chrono b/libcxx/include/chrono
index 209606929a649..dcdefd7d44dda 100644
--- a/libcxx/include/chrono
+++ b/libcxx/include/chrono
@@ -332,6 +332,9 @@ constexpr day  operator+(const day&  x, const days& y) noexcept;
 constexpr day  operator+(const days& x, const day&  y) noexcept;
 constexpr day  operator-(const day&  x, const days& y) noexcept;
 constexpr days operator-(const day&  x, const day&  y) noexcept;
+template<class charT, class traits>
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os, const day& d);
 
 // 25.8.4, class month    // C++20
 class month;
@@ -613,7 +616,13 @@ bool operator<(const time_zone& x, const time_zone& y) noexcept;
 bool operator>(const time_zone& x, const time_zone& y) noexcept;
 bool operator<=(const time_zone& x, const time_zone& y) noexcept;
 bool operator>=(const time_zone& x, const time_zone& y) noexcept;
+}  // chrono
+
+namespace std {
+  template<class charT> struct formatter<chrono::day, charT>;                     // C++20
+} // namespace std
 
+namespace chrono {
 // calendrical constants
 inline constexpr last_spec                              last{};       // C++20
 inline constexpr chrono::weekday                        Sunday{0};    // C++20
@@ -663,6 +672,7 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__chrono/calendar.h>
 #include <__chrono/convert_to_timespec.h>
+#include <__chrono/convert_to_tm.h>
 #include <__chrono/day.h>
 #include <__chrono/duration.h>
 #include <__chrono/file_clock.h>
@@ -686,6 +696,14 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 // standard-mandated includes
 #include <compare>
 
+#if !defined(_LIBCPP_HAS_NO_LOCALIZATION) && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT) && _LIBCPP_STD_VER > 17
+#  include <__chrono/formatter.h>
+#  include <__chrono/ostream.h>
+#  include <__chrono/parser_std_format_spec.h>
+#  include <__chrono/statically_widen.h>
+#endif
+
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif

diff  --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 4898daed6bfbf..cf1c7dd946d77 100644
--- a/libcxx/include/module.modulemap.in
+++ b/libcxx/include/module.modulemap.in
@@ -492,29 +492,43 @@ module std [system] {
     export *
 
     module __chrono {
-      module calendar              { private header "__chrono/calendar.h" }
-      module convert_to_timespec   { private header "__chrono/convert_to_timespec.h" }
-      module day                   { private header "__chrono/day.h" }
-      module duration              { private header "__chrono/duration.h" }
-      module file_clock            { private header "__chrono/file_clock.h" }
-      module hh_mm_ss              { private header "__chrono/hh_mm_ss.h" }
-      module high_resolution_clock {
+      module calendar               { private header "__chrono/calendar.h" }
+      module convert_to_timespec    { private header "__chrono/convert_to_timespec.h" }
+      module convert_to_tm          { private header "__chrono/convert_to_tm.h" }
+      module day                    { private header "__chrono/day.h" }
+      module duration               { private header "__chrono/duration.h" }
+      module file_clock             { private header "__chrono/file_clock.h" }
+      module formatter              {
+        @requires_LIBCXX_ENABLE_LOCALIZATION@
+        private header "__chrono/formatter.h"
+      }
+      module hh_mm_ss               { private header "__chrono/hh_mm_ss.h" }
+      module high_resolution_clock  {
         private header "__chrono/high_resolution_clock.h"
         export steady_clock
         export system_clock
       }
-      module literals              { private header "__chrono/literals.h" }
-      module month                 { private header "__chrono/month.h" }
-      module month_weekday         { private header "__chrono/month_weekday.h" }
-      module monthday              { private header "__chrono/monthday.h" }
-      module steady_clock          { private header "__chrono/steady_clock.h" }
-      module system_clock          { private header "__chrono/system_clock.h" }
-      module time_point            { private header "__chrono/time_point.h" }
-      module weekday               { private header "__chrono/weekday.h" }
-      module year                  { private header "__chrono/year.h" }
-      module year_month            { private header "__chrono/year_month.h" }
-      module year_month_day        { private header "__chrono/year_month_day.h" }
-      module year_month_weekday    { private header "__chrono/year_month_weekday.h" }
+      module literals               { private header "__chrono/literals.h" }
+      module month                  { private header "__chrono/month.h" }
+      module month_weekday          { private header "__chrono/month_weekday.h" }
+      module monthday               { private header "__chrono/monthday.h" }
+      module ostream                {
+        @requires_LIBCXX_ENABLE_LOCALIZATION@
+        private header "__chrono/ostream.h"
+      }
+      module parser_std_format_spec {
+        @requires_LIBCXX_ENABLE_LOCALIZATION@
+        private header "__chrono/parser_std_format_spec.h"
+      }
+      module statically_widen       { private header "__chrono/statically_widen.h" }
+      module steady_clock           { private header "__chrono/steady_clock.h" }
+      module system_clock           { private header "__chrono/system_clock.h" }
+      module time_point             { private header "__chrono/time_point.h" }
+      module weekday                { private header "__chrono/weekday.h" }
+      module year                   { private header "__chrono/year.h" }
+      module year_month             { private header "__chrono/year_month.h" }
+      module year_month_day         { private header "__chrono/year_month_day.h" }
+      module year_month_weekday     { private header "__chrono/year_month_weekday.h" }
     }
   }
   module codecvt {

diff  --git a/libcxx/test/libcxx/private_headers.verify.cpp b/libcxx/test/libcxx/private_headers.verify.cpp
index 0fc45a68c4f4f..58a7e40f666a4 100644
--- a/libcxx/test/libcxx/private_headers.verify.cpp
+++ b/libcxx/test/libcxx/private_headers.verify.cpp
@@ -238,15 +238,20 @@ END-SCRIPT
 #include <__charconv/to_chars_result.h> // expected-error@*:* {{use of private header from outside its module: '__charconv/to_chars_result.h'}}
 #include <__chrono/calendar.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/calendar.h'}}
 #include <__chrono/convert_to_timespec.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/convert_to_timespec.h'}}
+#include <__chrono/convert_to_tm.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/convert_to_tm.h'}}
 #include <__chrono/day.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/day.h'}}
 #include <__chrono/duration.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/duration.h'}}
 #include <__chrono/file_clock.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/file_clock.h'}}
+#include <__chrono/formatter.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/formatter.h'}}
 #include <__chrono/hh_mm_ss.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/hh_mm_ss.h'}}
 #include <__chrono/high_resolution_clock.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/high_resolution_clock.h'}}
 #include <__chrono/literals.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/literals.h'}}
 #include <__chrono/month.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/month.h'}}
 #include <__chrono/month_weekday.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/month_weekday.h'}}
 #include <__chrono/monthday.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/monthday.h'}}
+#include <__chrono/ostream.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/ostream.h'}}
+#include <__chrono/parser_std_format_spec.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/parser_std_format_spec.h'}}
+#include <__chrono/statically_widen.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/statically_widen.h'}}
 #include <__chrono/steady_clock.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/steady_clock.h'}}
 #include <__chrono/system_clock.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/system_clock.h'}}
 #include <__chrono/time_point.h> // expected-error@*:* {{use of private header from outside its module: '__chrono/time_point.h'}}

diff  --git a/libcxx/test/libcxx/transitive_includes/cxx20/expected.chrono b/libcxx/test/libcxx/transitive_includes/cxx20/expected.chrono
index 9f1521d4d4835..72678cfc80a10 100644
--- a/libcxx/test/libcxx/transitive_includes/cxx20/expected.chrono
+++ b/libcxx/test/libcxx/transitive_includes/cxx20/expected.chrono
@@ -1,10 +1,51 @@
+algorithm
+array
+atomic
+bit
+bitset
+cctype
+cerrno
+charconv
 climits
 cmath
 compare
+concepts
+cstdarg
 cstddef
 cstdint
+cstdio
+cstdlib
+cstring
 ctime
+cwchar
+cwctype
+exception
+format
+functional
+initializer_list
+ios
+iosfwd
+istream
+iterator
 limits
+locale
+memory
+mutex
+new
+optional
+ostream
 ratio
+sstream
+stdexcept
+streambuf
+string
+string_view
+system_error
+tuple
 type_traits
+typeinfo
+unordered_map
+utility
+variant
+vector
 version

diff  --git a/libcxx/test/libcxx/transitive_includes/cxx2b/expected.chrono b/libcxx/test/libcxx/transitive_includes/cxx2b/expected.chrono
index 9f1521d4d4835..4b73f94a4331d 100644
--- a/libcxx/test/libcxx/transitive_includes/cxx2b/expected.chrono
+++ b/libcxx/test/libcxx/transitive_includes/cxx2b/expected.chrono
@@ -1,10 +1,44 @@
+array
+atomic
+bit
+bitset
+cctype
+cerrno
+charconv
 climits
 cmath
 compare
+concepts
+cstdarg
 cstddef
 cstdint
+cstdio
+cstdlib
+cstring
 ctime
+cwchar
+cwctype
+exception
+format
+initializer_list
+ios
+iosfwd
+istream
 limits
+locale
+memory
+mutex
+new
+optional
+ostream
 ratio
+sstream
+stdexcept
+streambuf
+string
+string_view
+system_error
+tuple
 type_traits
+typeinfo
 version

diff  --git a/libcxx/test/libcxx/utilities/format/format.formatter/format.formatter.spec/formattable.compile.pass.cpp b/libcxx/test/libcxx/utilities/format/format.formatter/format.formatter.spec/formattable.compile.pass.cpp
index ab125ad03be74..177900b181cd4 100644
--- a/libcxx/test/libcxx/utilities/format/format.formatter/format.formatter.spec/formattable.compile.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.formatter/format.formatter.spec/formattable.compile.pass.cpp
@@ -122,6 +122,16 @@ void test_P0645() {
 // enabled.
 template <class CharT>
 void test_P1361() {
+// The chrono formatters require localization support.
+// [time.format]/7
+//   If the chrono-specs is omitted, the chrono object is formatted as if by
+//   streaming it to std::ostringstream os with the formatting
+//   locale imbued and copying os.str() through the output iterator of the
+//   context with additional padding and adjustments as specified by the format
+//   specifiers.
+// In libc++ std:::ostringstream requires localization support.
+#ifndef TEST_HAS_NO_LOCALIZATION
+
   assert_is_not_formattable<std::chrono::microseconds, CharT>();
 
   assert_is_not_formattable<std::chrono::sys_time<std::chrono::microseconds>, CharT>();
@@ -131,7 +141,7 @@ void test_P1361() {
   assert_is_not_formattable<std::chrono::file_time<std::chrono::microseconds>, CharT>();
   assert_is_not_formattable<std::chrono::local_time<std::chrono::microseconds>, CharT>();
 
-  assert_is_not_formattable<std::chrono::day, CharT>();
+  assert_is_formattable<std::chrono::day, CharT>();
   assert_is_not_formattable<std::chrono::month, CharT>();
   assert_is_not_formattable<std::chrono::year, CharT>();
 
@@ -156,6 +166,8 @@ void test_P1361() {
   //assert_is_formattable<std::chrono::local_info, CharT>();
 
   //assert_is_formattable<std::chrono::zoned_time, CharT>();
+
+#endif // TEST_HAS_NO_LOCALIZATION
 }
 
 // Tests for P1636 Formatters for library types

diff  --git a/libcxx/test/std/time/time.cal/time.cal.day/time.cal.day.nonmembers/ostream.pass.cpp b/libcxx/test/std/time/time.cal/time.cal.day/time.cal.day.nonmembers/ostream.pass.cpp
new file mode 100644
index 0000000000000..255f7a744e54c
--- /dev/null
+++ b/libcxx/test/std/time/time.cal/time.cal.day/time.cal.day.nonmembers/ostream.pass.cpp
@@ -0,0 +1,90 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// TODO FMT Investigate Windows issues.
+// UNSUPPORTED msvc, target={{.+}}-windows-gnu
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+// class day;
+
+// template<class charT, class traits>
+//   basic_ostream<charT, traits>&
+//     operator<<(basic_ostream<charT, traits>& os, const day& d);
+
+#include <chrono>
+#include <cassert>
+#include <sstream>
+
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "test_macros.h"
+
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+template <class CharT>
+static std::basic_string<CharT> stream_c_locale(std::chrono::day day) {
+  std::basic_stringstream<CharT> sstr;
+  sstr << day;
+  return sstr.str();
+}
+
+template <class CharT>
+static std::basic_string<CharT> stream_fr_FR_locale(std::chrono::day day) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_fr_FR_UTF_8);
+  sstr.imbue(locale);
+  sstr << day;
+  return sstr.str();
+}
+
+template <class CharT>
+static std::basic_string<CharT> stream_ja_JP_locale(std::chrono::day day) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_ja_JP_UTF_8);
+  sstr.imbue(locale);
+  sstr << day;
+  return sstr.str();
+}
+
+template <class CharT>
+static void test() {
+  using namespace std::literals::chrono_literals;
+
+  assert(stream_c_locale<CharT>(0d) == SV("00 is not a valid day"));
+  assert(stream_c_locale<CharT>(1d) == SV("01"));
+  assert(stream_c_locale<CharT>(31d) == SV("31"));
+  assert(stream_c_locale<CharT>(255d) == SV("255 is not a valid day"));
+
+  assert(stream_fr_FR_locale<CharT>(0d) == SV("00 is not a valid day"));
+  assert(stream_fr_FR_locale<CharT>(1d) == SV("01"));
+  assert(stream_fr_FR_locale<CharT>(31d) == SV("31"));
+  assert(stream_fr_FR_locale<CharT>(255d) == SV("255 is not a valid day"));
+
+  assert(stream_ja_JP_locale<CharT>(0d) == SV("00 is not a valid day"));
+  assert(stream_ja_JP_locale<CharT>(1d) == SV("01"));
+  assert(stream_ja_JP_locale<CharT>(31d) == SV("31"));
+  assert(stream_ja_JP_locale<CharT>(255d) == SV("255 is not a valid day"));
+}
+
+int main(int, char**) {
+  using namespace std::literals::chrono_literals;
+
+  test<char>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}

diff  --git a/libcxx/test/std/time/time.syn/formatter.day.pass.cpp b/libcxx/test/std/time/time.syn/formatter.day.pass.cpp
new file mode 100644
index 0000000000000..3dbc1e9d7b0cc
--- /dev/null
+++ b/libcxx/test/std/time/time.syn/formatter.day.pass.cpp
@@ -0,0 +1,127 @@
+//===----------------------------------------------------------------------===//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// TODO FMT Investigate Windows issues.
+// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+
+// template<class charT> struct formatter<chrono::day, charT>;
+
+#include <chrono>
+#include <format>
+
+#include <cassert>
+#include <concepts>
+#include <locale>
+#include <iostream>
+#include <type_traits>
+
+#include "formatter_tests.h"
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "string_literal.h"
+#include "test_macros.h"
+
+template <class CharT>
+static void test_no_chrono_specs() {
+  using namespace std::literals::chrono_literals;
+
+  // Valid day
+  check.template operator()<"{}">(SV("01"), 1d);
+  check.template operator()<"{:*^4}">(SV("*01*"), 1d);
+  check.template operator()<"{:*>3}">(SV("*01"), 1d);
+
+  // Invalid day
+  check.template operator()<"{}">(SV("00 is not a valid day"), 0d);
+  check.template operator()<"{:*^23}">(SV("*00 is not a valid day*"), 0d);
+}
+
+template <class CharT>
+static void test_valid_values() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr string_literal fmt{"{:%%d='%d'%t%%Od='%Od'%t%%e='%e'%t%%Oe='%Oe'%n}"};
+  constexpr string_literal lfmt{"{:L%%d='%d'%t%%Od='%Od'%t%%e='%e'%t%%Oe='%Oe'%n}"};
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check.template operator()<fmt>(SV("%d='00'\t%Od='00'\t%e=' 0'\t%Oe=' 0'\n"), 0d);
+  check.template operator()<fmt>(SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), 1d);
+  check.template operator()<fmt>(SV("%d='31'\t%Od='31'\t%e='31'\t%Oe='31'\n"), 31d);
+#if defined(_AIX)
+  check.template operator()<fmt>(SV("%d='55'\t%Od='55'\t%e='55'\t%Oe='55'\n"), 255d);
+#else
+  check.template operator()<fmt>(SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), 255d);
+#endif
+
+  // Use the global locale (fr_FR)
+  check.template operator()<lfmt>(SV("%d='00'\t%Od='00'\t%e=' 0'\t%Oe=' 0'\n"), 0d);
+  check.template operator()<lfmt>(SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), 1d);
+  check.template operator()<lfmt>(SV("%d='31'\t%Od='31'\t%e='31'\t%Oe='31'\n"), 31d);
+#if defined(_AIX)
+  check.template operator()<lfmt>(SV("%d='55'\t%Od='55'\t%e='55'\t%Oe='55'\n"), 255d);
+#else
+  check.template operator()<lfmt>(SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), 255d);
+#endif
+
+  // Use supplied locale (ja_JP). This locale has a 
diff erent alternate on some platforms.
+#if defined(__APPLE__) || defined(_AIX)
+  lcheck.template operator()<lfmt>(loc, SV("%d='00'\t%Od='00'\t%e=' 0'\t%Oe=' 0'\n"), 0d);
+  lcheck.template operator()<lfmt>(loc, SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), 1d);
+  lcheck.template operator()<lfmt>(loc, SV("%d='31'\t%Od='31'\t%e='31'\t%Oe='31'\n"), 31d);
+#  if defined(_AIX)
+  check.template operator()<fmt>(SV("%d='55'\t%Od='55'\t%e='55'\t%Oe='55'\n"), 255d);
+#  else
+  check.template operator()<fmt>(SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), 255d);
+#  endif
+#else  // defined(__APPLE__) || defined(_AIX)
+  lcheck.template operator()<lfmt>(loc, SV("%d='00'\t%Od='〇'\t%e=' 0'\t%Oe='〇'\n"), 0d);
+  lcheck.template operator()<lfmt>(loc, SV("%d='01'\t%Od='一'\t%e=' 1'\t%Oe='一'\n"), 1d);
+  lcheck.template operator()<lfmt>(loc, SV("%d='31'\t%Od='三十一'\t%e='31'\t%Oe='三十一'\n"), 31d);
+  lcheck.template operator()<lfmt>(loc, SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), 255d);
+#endif // defined(__APPLE__) || defined(_AIX)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test() {
+  using namespace std::literals::chrono_literals;
+
+  test_no_chrono_specs<CharT>();
+  test_valid_values<CharT>();
+  check_invalid_types<CharT>({SV("d"), SV("e"), SV("Od"), SV("Oe")}, 0d);
+
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:A"), 0d);
+  check_exception("The chrono-specs contains a '{'", SV("{:%%{"), 0d);
+  check_exception("End of input while parsing the modifier chrono conversion-spec", SV("{:%"), 0d);
+  check_exception("End of input while parsing the modifier E", SV("{:%E"), 0d);
+  check_exception("End of input while parsing the modifier O", SV("{:%O"), 0d);
+
+  // Precision not allowed
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:.3}"), 0d);
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}

diff  --git a/libcxx/test/std/time/time.syn/formatter_tests.h b/libcxx/test/std/time/time.syn/formatter_tests.h
new file mode 100644
index 0000000000000..3fe10e9c80f76
--- /dev/null
+++ b/libcxx/test/std/time/time.syn/formatter_tests.h
@@ -0,0 +1,269 @@
+//===----------------------------------------------------------------------===//
+// 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 TEST_STD_TIME_TIME_SYN_FORMATTER_TESTS_H
+#define TEST_STD_TIME_TIME_SYN_FORMATTER_TESTS_H
+
+#include "make_string.h"
+#include "string_literal.h"
+
+#include <algorithm>
+#include <cassert>
+#include <set>
+#include <string>
+#include <string_view>
+
+#define STR(S) MAKE_STRING(CharT, S)
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+template <class CharT>
+using format_context = std::conditional_t<std::same_as<CharT, char>, std::format_context, std::wformat_context>;
+#else
+template <class CharT>
+using format_context = std::format_context;
+#endif
+
+inline constexpr auto check = []<string_literal fmt, class CharT, class... Args>(
+    std::basic_string_view<CharT> expected, const Args&... args) constexpr {
+  std::basic_string<CharT> out = std::format(fmt.template sv<CharT>(), args...);
+  if constexpr (std::same_as<CharT, char>)
+    if (out != expected)
+      std::cerr << "\nFormat string   " << fmt.template sv<CharT>() << "\nExpected output " << expected
+                << "\nActual output   " << out << '\n';
+  assert(out == expected);
+};
+
+inline constexpr auto lcheck = []<string_literal fmt, class CharT, class... Args>(
+    const std::locale& loc, std::basic_string_view<CharT> expected, const Args&... args) constexpr {
+  std::basic_string<CharT> out = std::format(loc, fmt.template sv<CharT>(), args...);
+  if constexpr (std::same_as<CharT, char>)
+    if (out != expected)
+      std::cerr << "\nFormat string   " << fmt.template sv<CharT>() << "\nExpected output " << expected
+                << "\nActual output   " << out << '\n';
+  assert(out == expected);
+};
+
+inline constexpr auto check_exception =
+    []<class CharT, class... Args>(
+        [[maybe_unused]] std::string_view what,
+        [[maybe_unused]] std::basic_string_view<CharT> fmt,
+        [[maybe_unused]] const Args&... args) {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+      try {
+        TEST_IGNORE_NODISCARD std::vformat(fmt, std::make_format_args<format_context<CharT>>(args...));
+        if constexpr (std::same_as<CharT, char>)
+          std::cerr << "\nFormat string   " << fmt << "\nDidn't throw an exception.\n";
+        assert(false);
+      } catch (const std::format_error& e) {
+#  if defined(_LIBCPP_VERSION)
+        if constexpr (std::same_as<CharT, char>)
+          if (e.what() != what)
+            std::cerr << "\nFormat string   " << fmt << "\nExpected exception " << what << "\nActual exception   "
+                      << e.what() << '\n';
+        assert(e.what() == what);
+#  endif
+        return;
+      }
+      assert(false);
+#endif
+    };
+
+template <class CharT, class T>
+void check_invalid_type(const std::set<std::basic_string_view<CharT>>& valid_types,
+                        std::string_view what,
+                        std::basic_string<CharT> type,
+                        const T& arg) {
+  std::basic_string<CharT> fmt{STR("{:%") + type + STR("}")};
+
+  if (valid_types.contains(std::basic_string_view<CharT>{type})) {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    try {
+#endif
+      TEST_IGNORE_NODISCARD std::vformat(
+          std::basic_string_view<CharT>{fmt}, std::make_format_args<format_context<CharT>>(arg));
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    } catch (const std::format_error& e) {
+#  if defined(_LIBCPP_VERSION)
+      if constexpr (std::same_as<CharT, char>)
+        std::cerr << "\nFormat string        " << fmt << "\nUnexpected exception " << e.what() << '\n';
+#  endif
+      assert(false);
+    }
+#endif
+  } else {
+    check_exception(what, std::basic_string_view<CharT>{fmt}, arg);
+  }
+}
+
+template <class CharT, class T>
+void check_invalid_types(const std::set<std::basic_string_view<CharT>>& valid_types, const T& arg) {
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a weekday", STR("a"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a weekday", STR("A"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a month", STR("b"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a month", STR("B"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date and time", STR("c"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a year", STR("C"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a day", STR("d"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("D"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a day", STR("e"), arg);
+  // E - the modifier is checked separately
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("f"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("F"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("g"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("G"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a month", STR("h"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain an hour", STR("H"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("i"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain an hour", STR("I"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date or duration", STR("j"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("J"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("k"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("K"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("l"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("L"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a month", STR("m"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a minute", STR("M"), arg);
+  // n - valid
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("N"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("o"), arg);
+  // O - the modifier is checked separately
+  check_invalid_type(valid_types, "The supplied date time doesn't contain an hour", STR("p"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("P"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a duration", STR("q"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a duration", STR("Q"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time", STR("r"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time", STR("R"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("s"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a second", STR("S"), arg);
+  // t - valid
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time", STR("T"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a weekday", STR("u"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("U"), arg);
+  check_invalid_type(valid_types, "The date time type specifier is invalid", STR("v"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("V"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a weekday", STR("w"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("W"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("x"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time", STR("X"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a year", STR("y"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a year", STR("y"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time zone", STR("z"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time zone", STR("Z"), arg);
+
+  // *** E modifier
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ea"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EA"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Eb"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EB"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date and time", STR("Ec"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a year", STR("EC"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ed"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("ED"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ee"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EE"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ef"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EF"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Eg"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EG"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Eh"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EH"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ei"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EI"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ej"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EJ"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ek"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EK"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("El"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EL"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Em"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EM"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("En"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EN"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Eo"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EO"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ep"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EP"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Eq"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EQ"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Er"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("ER"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Es"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("ES"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Et"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("ET"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Eu"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EU"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ev"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EV"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("Ew"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EW"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("Ex"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time", STR("EX"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a year", STR("Ey"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a year", STR("EY"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time zone", STR("Ez"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("EZ"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier E is invalid", STR("E%"), arg);
+
+  // *** O modifier
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Oa"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OA"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Ob"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OB"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Oc"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OC"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a day", STR("Od"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OD"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a day", STR("Oe"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OE"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Of"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OF"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Og"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OG"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Oh"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain an hour", STR("OH"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Oi"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain an hour", STR("OI"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Oj"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OJ"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Ok"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OK"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Ol"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OL"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a month", STR("Om"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a minute", STR("OM"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("On"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("ON"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Oo"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OO"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Op"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OP"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Oq"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OQ"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Or"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OR"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Os"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a second", STR("OS"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Ot"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OT"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a weekday", STR("Ou"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("OU"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Ov"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("OV"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a weekday", STR("Ow"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a date", STR("OW"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("Ox"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OX"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a year", STR("Oy"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OY"), arg);
+  check_invalid_type(valid_types, "The supplied date time doesn't contain a time zone", STR("Oz"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("OZ"), arg);
+  check_invalid_type(valid_types, "The date time type specifier for modifier O is invalid", STR("O%"), arg);
+}
+
+#endif // TEST_STD_TIME_TIME_SYN_FORMATTER_TESTS_H

diff  --git a/libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/types.compile.pass.cpp b/libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/types.compile.pass.cpp
index 9bf871ecccb16..8beaeb2cfe472 100644
--- a/libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/types.compile.pass.cpp
+++ b/libcxx/test/std/utilities/format/format.formatter/format.formatter.spec/types.compile.pass.cpp
@@ -164,6 +164,16 @@ void test_P0645() {
 // enabled.
 template <class CharT>
 void test_P1361() {
+// The chrono formatters require localization support.
+// [time.format]/7
+//   If the chrono-specs is omitted, the chrono object is formatted as if by
+//   streaming it to std::ostringstream os with the formatting
+//   locale imbued and copying os.str() through the output iterator of the
+//   context with additional padding and adjustments as specified by the format
+//   specifiers.
+// In libc++ std:::ostringstream requires localization support.
+#ifndef TEST_HAS_NO_LOCALIZATION
+
   assert_formatter_is_disabled<std::chrono::microseconds, CharT>();
 
   assert_formatter_is_disabled<std::chrono::sys_time<std::chrono::microseconds>, CharT>();
@@ -173,7 +183,7 @@ void test_P1361() {
   assert_formatter_is_disabled<std::chrono::file_time<std::chrono::microseconds>, CharT>();
   assert_formatter_is_disabled<std::chrono::local_time<std::chrono::microseconds>, CharT>();
 
-  assert_formatter_is_disabled<std::chrono::day, CharT>();
+  assert_formatter_is_enabled<std::chrono::day, CharT>();
   assert_formatter_is_disabled<std::chrono::month, CharT>();
   assert_formatter_is_disabled<std::chrono::year, CharT>();
 
@@ -198,6 +208,8 @@ void test_P1361() {
   //assert_formatter_is_enabled<std::chrono::local_info, CharT>();
 
   //assert_formatter_is_enabled<std::chrono::zoned_time, CharT>();
+
+#endif // TEST_HAS_NO_LOCALIZATION
 }
 
 // Tests for P1636 Formatters for library types

diff  --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
index 491d82b1c524a..a5ec93559ad8e 100755
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -187,10 +187,12 @@ check-generated-output)
     # Depends on LC_COLLATE set at the top of this script.
     ! grep -rn '[^ -~]' libcxx/include libcxx/src libcxx/test libcxx/benchmarks \
            --exclude '*.dat' \
-           --exclude 'std_format_spec_string_unicode.bench.cpp' \
+           --exclude 'format_tests.h' \
+           --exclude 'formatter.day.pass.cpp' \
            --exclude 'grep.pass.cpp' \
            --exclude 'locale-specific_form.pass.cpp' \
-           --exclude 'format_tests.h' || false
+           --exclude 'std_format_spec_string_unicode.bench.cpp' \
+           || false
 
     # Reject code with trailing whitespace
     ! grep -rn '[[:blank:]]$' libcxx/include libcxx/src libcxx/test libcxx/benchmarks || false


        


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