From: Daniel Gutson <danielgutson@gmail.com>
To: musl@lists.openwall.com
Cc: Alexander Weps <exander77@pm.me>, Markus Wichmann <nullplan@gmx.net>
Subject: Re: [musl] Broken mktime calculations when crossing DST boundary
Date: Sun, 24 Mar 2024 00:32:00 -0300 [thread overview]
Message-ID: <CAFdMc-2DGdTJRnPYwyPP96T_830b2O29etqAK1gO_E7B=OMFPQ@mail.gmail.com> (raw)
In-Reply-To: <20240324020429.GU4163@brightrain.aerifal.cx>
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El sáb, 23 mar 2024, 23:04, Rich Felker <dalias@libc.org> escribió:
> 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.
>
I was so curious about this that I asked Perplexity for an example:
"For example, in the case of Samoa as mentioned in the search results, on
December 30, 2011, the time went from 23:59:59 on December 29 straight to
00:00:00 on December 31, effectively skipping December 30 entirely as the
country moved west of the International Date Line and also changed its time
zone from UTC-11 to UTC+13"
(
https://www.caktusgroup.com/blog/2019/03/21/coding-time-zones-and-daylight-saving-time/
)
This is sadly entertaining.
> 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.
>
> Rich
>
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next prev parent reply other threads:[~2024-03-24 3:32 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-22 19:56 Alexander Weps
2024-03-23 6:41 ` Markus Wichmann
[not found] ` <528SeRFaPfDw7fA4kqKDlio1U4RB_t9nmUemPcWw9_t1e2hBDpXYFmOqxAC37szgYvAVtmTuXWsmT64SSN3cSQFVdrQqXUAgkdTMPZQ0bg0=@pm.me>
2024-03-23 10:38 ` Markus Wichmann
2024-03-23 11:59 ` Alexander Weps
2024-03-23 12:00 ` Alexander Weps
2024-03-23 12:31 ` Rich Felker
2024-03-23 13:49 ` Alexander Weps
2024-03-23 15:31 ` Rich Felker
2024-03-23 16:54 ` Alexander Weps
2024-03-23 18:57 ` Alexander Weps
2024-03-23 19:33 ` Alexander Weps
2024-03-23 20:18 ` Rich Felker
2024-03-23 20:40 ` Alexander Weps
2024-03-24 0:36 ` Eric Pruitt
2024-03-24 2:04 ` Rich Felker
2024-03-24 3:32 ` Daniel Gutson [this message]
2024-03-24 11:05 ` Alexander Weps
2024-03-24 13:24 ` Alexander Weps
2024-03-23 12:01 ` Alexander Weps
2024-03-24 13:36 Alexander Weps
2024-03-24 16:59 ` Alexander Weps
2024-03-24 17:04 ` Rich Felker
2024-03-24 17:12 ` Alexander Weps
2024-03-24 18:00 ` Alexander Weps
2024-03-24 18:02 ` Rich Felker
2024-03-24 18:16 ` Alexander Weps
2024-03-24 18:24 ` Rich Felker
2024-03-24 18:36 ` Alexander Weps
2024-03-24 19:01 ` Joakim Sindholt
2024-03-24 19:05 ` Alexander Weps
2024-03-24 19:06 ` Alexander Weps
2024-03-24 19:13 ` Alexander Weps
2024-03-24 19:13 ` Alexander Weps
2024-03-24 19:22 ` Rich Felker
2024-03-24 19:57 ` Alexander Weps
2024-03-24 20:22 ` Rich Felker
2024-03-24 20:50 ` Alexander Weps
2024-03-24 21:43 ` Alexander Weps
2024-03-24 23:51 ` Thorsten Glaser
2024-03-25 0:36 ` Alexander Weps
2024-03-25 11:52 ` Alexander Weps
2024-03-25 12:21 ` Rich Felker
2024-03-25 12:55 ` Alexander Weps
2024-03-25 13:08 ` Rich Felker
2024-03-25 13:13 ` Alexander Weps
2024-03-25 13:13 ` Rich Felker
2024-03-25 13:24 ` Alexander Weps
2024-03-25 13:42 ` Rich Felker
2024-03-25 13:48 ` Alexander Weps
2024-03-25 13:50 ` Alexander Weps
2024-03-25 18:02 ` Rich Felker
2024-03-25 18:28 ` Alexander Weps
2024-03-25 18:53 ` Rich Felker
2024-03-25 18:57 ` Alexander Weps
2024-03-25 19:38 ` Rich Felker
2024-03-25 19:47 ` Rich Felker
2024-03-25 20:05 ` Alexander Weps
2024-03-25 20:12 ` Alexander Weps
2024-03-25 20:00 ` Alexander Weps
2024-03-25 20:23 ` Rich Felker
2024-03-25 20:31 ` Rich Felker
2024-03-25 23:19 ` Thorsten Glaser
2024-03-25 23:16 ` Thorsten Glaser
2024-03-25 13:44 ` Alexander Weps
2024-03-25 22:40 ` Thorsten Glaser
2024-03-25 22:59 ` Alexander Weps
2024-03-25 23:34 ` Thorsten Glaser
2024-03-26 12:45 ` Alexander Weps
2024-03-26 21:59 ` Thorsten Glaser
2024-03-27 0:14 ` Alexander Weps
2024-03-27 0:38 ` Alexander Weps
2024-03-27 1:35 ` Thorsten Glaser
2024-03-27 2:45 ` Alexander Weps
2024-03-27 4:42 ` Thorsten Glaser
2024-03-26 18:56 ` Alexander Weps
2024-03-25 23:13 ` Rich Felker
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