<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/62082>62082</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++][format][chrono] Subsecond part of `duration` is not correctly formatted
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          frederick-vs-ja
      </td>
    </tr>
</table>

<pre>
    Example
```C++
#include <chrono>
#include <cstdint>
#include <format>
#include <iostream>

int main()
{
    std::cout << std::format("{0:%M:%S} {0}\n", std::chrono::duration<int, std::ratio<101, 103>>{40});
 std::cout << std::format("{0:%M:%S} {0}\n", std::chrono::duration<int, std::ratio<1, 1024>>{1511});
}
```

Current output  ([Godbolt link](https://godbolt.org/z/PcPd8j1PW)):
```
00:39.000023 40[101/103]s
00:01.0000000487 1511[1/1024]s
```

Expected output:
```
00:39.223300 40[101/103]s
00:01.4755859375 1511[1/1024]s
```

It seems that we should convert (via `duration_cast`) `__fraction` to `precision` before writing its `count()`.
https://github.com/llvm/llvm-project/blob/9d4df97ff0ef459316cdb55f17eaea9fe9b1c1d5/libcxx/include/__chrono/formatter.h#L87

https://github.com/llvm/llvm-project/blob/9d4df97ff0ef459316cdb55f17eaea9fe9b1c1d5/libcxx/include/__chrono/formatter.h#L101-L109
</pre>
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