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Date: Mon, 25 Mar 2024 13:24:57 +0000
From: Alexander Weps <exander77@...me>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: Broken mktime calculations when crossing DST boundary
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.
>
> What did you do that got glibc to output 2012-01-01? I guess you wrote
> code to do some wacky arithmetic after the original code you already
> had, rather than changing the code to start with 2011-12-31 as I
> suggested to get a look at what's happening.
>
> > > In any case, the core issue you're hitting here is that time zones are
> > > HARD to work with and that there is inherent complexity that libc
> > > cannot save you from. You only got lucky that what you were trying to
> > > do "worked" with glibc because you were iterating days forward; if you
> > > were doing reverse, it would break exactly the same way.
> >
> > I am not really commenting on this, until you sort out the above
> > inconsistencies.
>
>
> I already have but you refuse to look.
It was addressed, do didn't scroll at the end of the e-mail.
>
> Rich
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