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From: "howard.hinnant at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/114260]
 std::formatter<std::chrono::utc_time<std::chrono::days>> formats as the
 previous day
Date: Thu, 07 Mar 2024 22:44:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114260

--- Comment #2 from Howard Hinnant <howard.hinnant at gmail dot com> ---
This:

2024-03-05 00:00:00
2024-03-04 23:59:33

looks like correct output to me.

sys_time and utc_time map to the same civil calendar date/time (except duri=
ng a
leap second).  That is 2024-03-05 00:00:00 sys_time, converted to utc_time,
also should print out as 2024-03-05 00:00:00.

The only difference between sys_time and utc_time is that utc_time counts t=
he
leap seconds since 1970 (27 at this point).  This means if you look at the
difference in .time_since_epoch(), utc_time will be 27 seconds longer, even
though it prints out as the same date and time.

Consequently, 2024-03-05 00:00:00 in utc_time is *not* a multiple of 86400s,
but rather 27s greater than a multiple of 86400s.  And all round<days>(uday=
s)
does is round the .time_since_epoch() to the nearest multiple of 86400s.  W=
hich
in utc_time is 27s earlier, or 2024-03-04 23:59:33.

So in summary, if you make a change, and *don't* get this output, then you'=
ve
introduced a bug.=