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Date: Mon, 25 Mar 2024 14:02:08 -0400
From: Rich Felker <dalias@...c.org>
To: Alexander Weps <exander77@...me>
Cc: musl@...ts.openwall.com
Subject: Re: Broken mktime calculations when crossing DST boundary
On Mon, Mar 25, 2024 at 09:42:53AM -0400, Rich Felker wrote:
> On Mon, Mar 25, 2024 at 01:24:57PM +0000, Alexander Weps wrote:
> > See below.
> >
> > AW
> >
> >
> > On Monday, March 25th, 2024 at 14:13, Rich Felker <dalias@...c.org> wrote:
> >
> > > On Mon, Mar 25, 2024 at 12:55:28PM +0000, Alexander Weps wrote:
> > >
> > > > > If you take your test program and switch it to initialize with
> > > > > tm_mday=31, then do -=1 instead of +=1, you'll find that it gives
> > > > > 2011-12-29 01:00:00 -10 as well, only now it seems like the correct,
> > > > > expected thing to happen. Any change to "fix" the case you're
> > > > > complaining about would necessarily break this case.
> > > >
> > > > So (- day, +day):
> > > >
> > > > Musl:
> > > > 2011-12-31 01:00:00 +14
> > > > 2011-12-29 01:00:00 -10
> > > > 2011-12-29 01:00:00 -10
> > > >
> > > > Glibc:
> > > > 2012-01-01 01:00:00 +14
> > > > 2011-12-31 01:00:00 +14
> > > > 2012-01-01 01:00:00 +14
> > > >
> > > > Seems like musl doesn't even interpret the initial struct tm
> > > > correctly in that case. It is off by day.
> > > >
> > > > Because December only had 30 days, 31s day after normalization is
> > > > January 1st.
> > >
> > >
> > > This is nonsense. December has a day 31, which you can clearly see
> > > from the glibc output. For this particular year in this zone, with the
> > > zone rule change, there are "only 30 days" in December, but they are
> > > numbered 1-29 and 31, not 1-30.
> >
> > You confuse day of month which is represented in tm_mday with
> > calendar day that is interpreted by strftime.
> >
> > You said to set tm_mday = 31, which would be January 1st after normalization.
> > December 31s is 30th day of month represented as tm_mday = 30.
>
> OK, I meant tm_mday=31-1.
Um, no, where did you get that idea? I just assumed you were right
because I always forget which tm_* are off-by-1, but tm_mday is
one-based not zero-based:
int tm_mday; // day of the month -- [1, 31]
(per the standard). So how did you end up getting the wrong thing? Are
you even running the code you say you are?
Rich
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