<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63912>63912</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `<chrono>`: `month::operator++/--` implementation is wrong
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          hewillk
      </td>
    </tr>
</table>

<pre>
    https://github.com/llvm/llvm-project/blob/d71329773d9ac0ba8cf570dc0d5ad89ce20a00c2/libcxx/include/__chrono/month.h#L34-L37

According to [[time.cal.month.members]](https://eel.is/c++draft/time.cal.month.members), the result after `month` increment should be in [1, 12].
```cpp
#include <chrono>

// only libc++ failed
static_assert(
  [] {
    auto m = std::chrono::December;
 ++m;
    return m.ok();
 }()
);
```
https://godbolt.org/z/z1d3oM86f

</pre>
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