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Date: Sat, 23 Mar 2024 16:18:04 -0400
From: Rich Felker <dalias@...c.org>
To: Alexander Weps <exander77@...me>
Cc: musl@...ts.openwall.com, Markus Wichmann <nullplan@....net>
Subject: Re: Broken mktime calculations when crossing DST boundary
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
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