On Fri, 7 Mar 2025, 05:39 Stewart Becker via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Would an alternative be to provide the conversion to system_clock from
> of __DATE__ and __TIME__ in a compile-time manner? Compilers that use
> local time for __DATE__ and __TIME__ also know their offset from UTC.
> The part that is missing is communicating this information to the language.
>
> I appreciate that __DATE__ and __TIME__ are C macros which C++ adopts,
> rather than C++ outright, so this may not be the right place to propose
> a __TIMEZONE__ macro.


I think you want an offset from UTC rather than a time since, otherwise you
need to determine the offset, which needs chrono::tzdb to be usable in
constexpr too!

  However, an alternative that would lie within
> C++'s remit would be a compile_time_clock that is required to provide:
>
> full constexpr usage, including now(),
> time_points that are equivalent to __DATE__and __TIME__, and
> the ability to cast its time_points to system_clock's / utc_clock's
> via clock_cast.
>
> Those who need reproducibility can achieve it without any extra work, as
> the existing approaches for __DATE__ and __TIME__ would be directly
> passed on to compile_time_clock.
>
> Stewart
>
> On 07/03/2025 01:58, Howard Hinnant via Std-Proposals wrote:
> > If we could change the C++ standard to say that __DATE__ and __TIME__
> are UTC, then the user could write this clock themselves. And that
> user-written clock could even return a system_clock::time_point.
> >
> > On macOS __DATE__ and __TIME__ are in the local time zone. And that
> blows a user-written clock out of the water unless the user can somehow
> inject the current local UTC offset into the build system.
> >
> > The advantage of basing it to on a UTC __DATE__ and __TIME__ is that the
> existing infrastructure for manipulating __DATE__ and __TIME__ remains and
> could manipulate this chrono::time_point.
> >
> > Howard
> >
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