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Date: Sat, 23 Mar 2024 22:04:29 -0400
From: Rich Felker <>
To: Alexander Weps <>
Cc:, Markus Wichmann <>
Subject: Re: Broken mktime calculations when crossing DST boundary

On Sat, Mar 23, 2024 at 08:40:50PM +0000, Alexander Weps wrote:
> Yes, the behavior is the same here glibc and musl and it can't
> reliably determine start of the day etc. Which is I assume expected.
> That's why there is tm_isdst = -1.
> I don't see any reliable way to determine beginning of the day without it.

It's rather inherent to the horribleness of DST that determining the
"beginning of the day" is not easy. In fact, it might not even be
well-defined, depending on where your timezone puts its transition (it
could happen right at midnight just to be evil; that it doesn't is
only a matter of polite convention).

> If I want to get beginning of the day I do it this way:
> before: 2010-10-31 14:00:00 CET
> tm_sec: 0
> tm_min: 0
> tm_hour: 14
> tm_mday: 31
> tm_mon: 9
> tm_year: 110
> tm_wday: 0
> tm_yday: 303
> tm_isdst: 0
> tm_gmtoff: 3600
> tm_zone: CET
> tm.tm_isdst = -1; <-- setting tm_isdst = -1
> tm.tm_hour = 0;
> mktime(&tm);
> after: 2010-10-31 00:00:00 CEST
> tm_sec: 0
> tm_min: 0
> tm_hour: 0
> tm_mday: 31
> tm_mon: 9
> tm_year: 110
> tm_wday: 0
> tm_yday: 303
> tm_isdst: 1
> tm_gmtoff: 7200
> tm_zone: CEST
> Is there a way, how to reliable get beginning of day etc. without
> tm_isdst = -1.

Depending on how you want to define it, yes, but it may need a second
call to mktime. First, call mktime with 00:00:00 and tm_isdst=0. If
the result has tm_isdst==0, you're done. If not, try again with the
original struct tm input but tm_isdst changed to 1. The only way this
procedure will fail is if the time 00:00:00 *does not exist* on that
particular day.

Note that if DST ends such that there are two times 00:00:00 on a
particular day, this will pick the second (non-DST) one, as the first
one might only be the new day temporarily, then jump back to the old
day, which does not strike me as a good "beginning of the day". You
could flip the order you try them around if you prefer to count it.


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