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Date: Sun, 24 Mar 2024 18:00:06 +0000
From: Alexander Weps <exander77@...me>
To: musl@...ts.openwall.com
Cc: Daniel Gutson <danielgutson@...il.com>, Markus Wichmann <nullplan@....net>
Subject: Re: Broken mktime calculations when crossing DST boundary
So I started from:
1900-01-01 00:00:00
Adding 59 seconds each cycle (to speed things up).
And I already have some cycles in Musl even for Europe/Prague.
1946-12-01 02:59:17 CET
1946-12-01 01:00:16 CET <-- There should be: 1946-12-01 02:00:16 GMT
This looks like an issue involving negative DST change.
There were 3 STD/DST changes in 1946 in Europe/Prague:
1945-10-01 01:00:00Z +01:00:00 standard CET
1946-05-06 01:00:00Z +02:00:00 daylight CEST
1946-10-06 01:00:00Z +01:00:00 standard CET
1946-12-01 02:00:00Z +00:00:00 daylight GMT
1947-02-23 02:00:00Z +01:00:00 standard CET
And it works in glibc. I was able to run it all the way from 1900 to 2200.
Glibc correctly went from CET to GMT and then back:
1946-12-01 02:59:57 CET
1946-12-01 02:00:56 GMT
...
AW
On Sunday, March 24th, 2024 at 18:12, Alexander Weps <exander77@...me> wrote:
> Adding seconds cannot make time go backwards. If that is how it was interpreted, than the interpretation is erroneous.
>
> I think I will do some test.
>
> If you start with 1900-01-01 00:00:00 and start adding seconds and calling mktime, you should reliable get to 2200-12-31 59:59:59 in all timezones. Same goes for minutes, hours, days, years.
>
> If that's not the case then the interpretation in musl is wrong.
>
> This is a basic elementary functionality of mktime.
>
> There can't be any going back in time a cycles.
>
> AW
>
>
>
> On Sunday, March 24th, 2024 at 18:04, Rich Felker dalias@...c.org wrote:
>
> > On Sun, Mar 24, 2024 at 01:36:42PM +0000, Alexander Weps wrote:
> >
> > > Also thanks for the Pacific/Apia example. Not only that it fails for that date:
> > > Pattern: * * * * * *
> > > Initial: 2011-12-29_23:59:59
> > > Expected: 2011-32-31_00:00:00
> > > Actual: 2011-12-29_00:00:00
> > > My cron tool again going back in time.
> > >
> > > It fails one other test.
> > >
> > > I have to run my tests on multiple timezones.
> > >
> > > And it works in glibc.
> > >
> > > And that's after I removed tm_isdst and rewrote half the code to accommodate.
> > >
> > > Can We agree on some simple premise that with no uncertain STD/DST
> > > settings (tm_isdst = 0 or tm_isdst = 1), incrementing seconds by 1
> > > and calling mktime should never cause time to go back?
> > >
> > > Well, behold Pacific/Apia:
> > >
> > > I set 2011-12-29 23:59:59:
> > >
> > > tm_sec: 59
> > > tm_min: 59
> > > tm_hour: 23
> > > tm_mday: 29
> > > tm_mon: 11
> > > tm_year: 111
> > > tm_wday: 0
> > > tm_yday: 0
> > > tm_isdst: 1
> > > tm_gmtoff: 0
> > > tm_zone: (null)
> > >
> > > Calling mktime to see if everything is correct:
> > > mktime(&tm);
> > >
> > > before: 2011-12-29 23:59:59 -10
> > > tm_sec: 59
> > > tm_min: 59
> > > tm_hour: 23
> > > tm_mday: 29
> > > tm_mon: 11
> > > tm_year: 111
> > > tm_wday: 4
> > > tm_yday: 362
> > > tm_isdst: 1
> > > tm_gmtoff: -36000
> > > tm_zone: -10
> > >
> > > Incrementing seconds and calling mktime:
> > > tm.tm_sec += 1;
> > > mktime(&tm);
> > >
> > > after: 2011-12-29 00:00:00 -10
> > > tm_sec: 0
> > > tm_min: 0
> > > tm_hour: 0
> > > tm_mday: 29
> > > tm_mon: 11
> > > tm_year: 111
> > > tm_wday: 4
> > > tm_yday: 362
> > > tm_isdst: 1
> > > tm_gmtoff: -36000
> > > tm_zone: -10
> > > We went from:
> > > 2011-12-29 23:59:59 -10
> > > To:
> > > 2011-12-29 00:00:00 -10
> > >
> > > By adding 1 second. The tm_isdst was not set to -1.
> > >
> > > This is totally unreliable.
> >
> > From what I understand, you've set the input to a time that doesn't
> > exist in the local timezone: 2011-12-30 00:00:00. This could be either
> > 1 second after 2011-12-29 23:59:59, as you intended, or 1 day before
> > 2011-12-31 00:00:00. The latter is how it was interpreted. However, it
> > does not seem to have correctly set tm_gmtoff to reflect how it was
> > interpreted. We should check this out because that's probably an
> > actual bug.
> >
> > Rich
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