From: Alexander Weps <exander77@pm.me>
To: musl@lists.openwall.com
Cc: Markus Wichmann <nullplan@gmx.net>
Subject: Re: [musl] Broken mktime calculations when crossing DST boundary
Date: Sat, 23 Mar 2024 20:40:50 +0000 [thread overview]
Message-ID: <6rPmS4jev9oFFibDdpKZ0mVRWLV3rhRcfqaUKu-5lPP7lc8zz8UH_kgPFA4r8wh1-qrYKfzVBpB0Ss6z4VMANASybmKZMeB9i3W3auYfhMI=@pm.me> (raw)
In-Reply-To: <20240323201804.GT4163@brightrain.aerifal.cx>
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.
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.
AW
On Saturday, March 23rd, 2024 at 21:18, Rich Felker <dalias@libc.org> wrote:
> On Sat, Mar 23, 2024 at 06:57:21PM +0000, Alexander Weps wrote:
>
> > So, in the meantime, I was debugging with not setting tm_isdst = -1;
> >
> > This causes pretty annoying behavior:
> >
> > before: 2010-10-31 14:00:00
> >
> > tm_sec: 0
> > tm_min: 0
> > tm_hour: 14
> > tm_mday: 31
> > tm_mon: 9
> > tm_year: 110
> > tm_wday: 0
> > tm_yday: 0
> > tm_isdst: 0
> > tm_gmtoff: 3600
> > tm_zone: CET
> >
> > tm->tm_hour = 0; <-- reset hour field
> > mktime(&tm);
> >
> > after: 2010-10-31 01:00:00 CEST <-- 10:00:00 instead of 00:00:00
>
>
> I guess you meant 01:00:00 not 10:00:00. This is expected. You asked
> mktime to normalize a time expressed in standard (non-DST, CET) time
> but referring to a time at which DST is in effect. After
> normalization, it expresses that time in DST (CEST). Since there is no
> tm_isdst<0 (the only source of arbitrary implementation choices)
> involved, you will find glibc and all other implementations do exactly
> the same thing here.
>
> > tm_sec: 0
> > tm_min: 0
> > tm_hour: 1
> > tm_mday: 31
> > tm_mon: 9
> > tm_year: 110
> > tm_wday: 0
> > tm_yday: 303
> > tm_isdst: 1
> > tm_gmtoff: 7200
> > tm_zone: CEST
> >
> > tm->tm_hour = 0;
> > mktime(&tm);
> >
> > after: 2010-10-31 00:00:00 CEST <-- second run gives a correct value
> > 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
> >
> > This basically means that setting field twice produces different
> > value each time:
>
>
> No it does not. After the first time, tm_isdst is 1. Now when you
> change the hour to 0, you are giving it a time expressed in DST. Since
> DST is in effect at this time, it's already normalized, and you get
> back what you put in.
>
> Rich
next prev parent reply other threads:[~2024-03-23 20:41 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 [this message]
2024-03-24 0:36 ` Eric Pruitt
2024-03-24 2:04 ` Rich Felker
2024-03-24 3:32 ` Daniel Gutson
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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