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 19:33:16 +0000 [thread overview]
Message-ID: <LunoDUNUaNH0ht3nRaDyHe_Z_AmZfBsVb4mkvSONeBW8KX-vY_ydPot97pzPIpTVRiGWprLS0l62TPP2xajaxBV4kKLw5hyKbK_NV3shaXQ=@pm.me> (raw)
In-Reply-To: <sIwTnW_WX_mb_WDtWrOqq_qJezwVq1O2KHLhy0rC7eStZYZp4tdGaz_SVxb3wnuNwc_DIZWaFEqU-UmdtvxeFPMxK0o6yUH0dLGcLm4TR4k=@pm.me>
What is the reliable way to get start of this day and similar?
Seems there is just no way if you say tm_isdst = -1 is not a proper way?
For cron calculations I have to move between seconds, minutes, hours, days, months etc. reliably.
AW
On Saturday, March 23rd, 2024 at 19:57, Alexander Weps <exander77@pm.me> 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
> 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:
>
> AW
>
>
>
>
> On Saturday, March 23rd, 2024 at 17:54, Alexander Weps exander77@pm.me wrote:
>
> > One of the main purposes of struct tm is to calculate date and time, by adding and substracting it's fields.
> >
> > > mktime cannot tell whether your non-normalized input was the result of
> > > you starting with 01:00:02 and adding 1 hour (in which case, our
> > > output does not reflect your intent) or of you starting with 3:00:02
> > > and subtracting 1 hour (in which case, our output does reflect your
> > > intent).
> >
> > We are not adding hours here, your example is completely unrelated.
> >
> > We are adding or subtracting minutes that changes hours.
> >
> > tm_sec: 2
> > tm_min: 60
> > tm_hour: 1
> >
> > vs
> >
> > tm_sec: 2
> > tm_min: 0
> > tm_hour: 2
> >
> > And:
> >
> > tm_sec: 2
> > tm_min: 59
> > tm_hour: 1
> >
> > vs
> >
> > tm_sec: 2
> > tm_min: -1
> > tm_hour: 2
> >
> > AW
> >
> > On Saturday, March 23rd, 2024 at 16:31, Rich Felker dalias@libc.org wrote:
> >
> > > On Sat, Mar 23, 2024 at 01:49:48PM +0000, Alexander Weps wrote:
> > >
> > > > I don't think time can go backwards by incrementing field under any conditions.
> > > >
> > > > Going from:
> > > > tm_sec: 2
> > > > tm_min: 60
> > > > tm_hour: 1
> > > > tm_mday: 31
> > > > tm_mon: 2
> > > > tm_year: 124
> > > > tm_wday: 0
> > > > tm_yday: 90
> > > > tm_isdst: -1
> > > >
> > > > To:
> >
> > tm_sec: 1
> > tm_min: 59
> > tm_hour: 2
> >
> > > > tm_sec: 2
> > > > tm_min: 0
> > > > tm_hour: 1
> > > > tm_mday: 31
> > > > tm_mon: 2
> > > > tm_year: 124
> > > > tm_wday: 0
> > > > tm_yday: 90
> > > > tm_isdst: 0
> > > >
> > > > Seems to be plain wrong. I cannot come up with any argument for this
> > > > being correct under any conditions.
> > >
> > > The above broke-down time is 2:00:02, which does not exist on that day
> > > as a normalized time. If interpreted as non-DST, it would be just a
> > > couple seconds past the end of non-DST (1:59:59.99999..). If
> > > interpreted as DST, it would be just under an hour before the start of
> > > DST (3:00:00), which, after normalization, is 1:00:02 non-DST.
> > >
> > > mktime cannot tell whether your non-normalized input was the result of
> > > you starting with 01:00:02 and adding 1 hour (in which case, our
> > > output does not reflect your intent) or of you starting with 3:00:02
> > > and subtracting 1 hour (in which case, our output does reflect your
> > > intent).
> > >
> > > > mktime was given a struct tm with uncertain STD/DST, it deduced it
> > > > is STD and then thrown away 60 minute information. The minutes got
> > > > reset from 60 to 0 and no other change was done.
> > >
> > > It did not deduce it was STD. It deduced it was non-normalized DST
> > > rather than non-normalized STD (this is an arbitrary choice), then
> > > normalized it and got STD.
> > >
> > > Rich
next prev parent reply other threads:[~2024-03-23 19:33 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 [this message]
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
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='LunoDUNUaNH0ht3nRaDyHe_Z_AmZfBsVb4mkvSONeBW8KX-vY_ydPot97pzPIpTVRiGWprLS0l62TPP2xajaxBV4kKLw5hyKbK_NV3shaXQ=@pm.me' \
--to=exander77@pm.me \
--cc=musl@lists.openwall.com \
--cc=nullplan@gmx.net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.vuxu.org/mirror/musl/
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).