Re: [musl] Broken mktime calculations when crossing DST boundary

[Rich Felker <dalias@libc.org>]

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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@libc.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.

> > 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.

Run the attached passing the starting date to check as the first/only
argument, and these test dates:

- "2011-12-29 00:00:00"
- "2011-12-31 00:00:00"

Hopefully that will clarify things for you. On musl you will see:

normalized input: 2011-12-29 00:00:00 -10
+1day per mktime: 2011-12-29 00:00:00 -10
+1day via time_t: 2011-12-31 00:00:00 +14
-1day per mktime: 2011-12-28 00:00:00 -10
-1day via time_t: 2011-12-28 00:00:00 -10

normalized input: 2011-12-31 00:00:00 +14
+1day per mktime: 2012-01-01 00:00:00 +14
+1day via time_t: 2012-01-01 00:00:00 +14
-1day per mktime: 2011-12-29 00:00:00 -10
-1day via time_t: 2011-12-29 00:00:00 -10

You can see what you get on glibc.

Rich

[-- Attachment #2: mktime_rel.c --]
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#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <errno.h>

int main(int argc, char **argv)
{
	struct tm tm = { .tm_isdst = -1 };

	if (!strptime(argv[1], "%Y-%m-%d %H:%M:%S", &tm)) {
		perror("strptime");
		return 1;
	}
	errno = 0;
	time_t t = mktime(&tm);
	if (t==-1 && errno) {
		perror("mktime");
		return 1;
	}

	char buf[100];
	strftime(buf, sizeof buf, "%Y-%m-%d %H:%M:%S %Z", &tm);
	printf("normalized input: %s\n", buf);

	struct tm tm2 = tm;
	tm2.tm_mday++;
	errno = 0;
	if (mktime(&tm2)==-1 && errno) {
		perror("mktime day+1");
	} else {
		strftime(buf, sizeof buf, "%Y-%m-%d %H:%M:%S %Z", &tm2);
		printf("+1day per mktime: %s\n", buf);
	}
	localtime_r(&(time_t){t+86400}, &tm2);
	strftime(buf, sizeof buf, "%Y-%m-%d %H:%M:%S %Z", &tm2);
	printf("+1day via time_t: %s\n", buf);

	tm2 = tm;
	tm2.tm_mday--;
	errno = 0;
	if (mktime(&tm2)==-1 && errno) {
		perror("mktime day-1");
	} else {
		strftime(buf, sizeof buf, "%Y-%m-%d %H:%M:%S %Z", &tm2);
		printf("-1day per mktime: %s\n", buf);
	}
	localtime_r(&(time_t){t-86400}, &tm2);
	strftime(buf, sizeof buf, "%Y-%m-%d %H:%M:%S %Z", &tm2);
	printf("-1day via time_t: %s\n", buf);
}