[Remind-Fans] Remind 04.02.00: make test failed on OpenBSD

[Martin Ziemer]

Am Mon, Oct 17, 2022 at 09:31:45AM -0400 schrieb Dianne Skoll via Remind-fans:
> On Mon, 17 Oct 2022 13:25:13 +0000
> Tim Chase via Remind-fans <remind-fans at lists.skoll.ca> wrote:
> > I just compared the clocks on my OpenBSD machines using
> > 
> >   $ TZ=America/Toronto date
> >   Mon Oct 17 09:15:24 EDT 2022
> Yes, but that's not the test we need.  We need to know what time
> it thinks it is at 2022-11-06T01:59 in Toronto.
> 
> Daylight Saving Time in Toronto ends at 2022-11-06T02:00 local time.
> However, there's an ambiguity and it's possible OpenBSD's C library
> and glibg handle it differently.  There are actually *two different*
> times in Toronto at 2022-11-06T01:59:  One before daylight saving time
> ends, which corresponds to the time *before* DST ends, which means
> 2022-11-06T05:59 UTC.  The second 2022-11-06T01:59 corresponds to
> the time *after* DST ends, which is 2022-11-06T06:59.

I just tested this case and found out, with negative tm_isdst (as used in 
the code) the daylight saving changes from 01:11 to 01:12!: 
	(57) horrad at dozer:~/Temp $ TZ=America/Toronto date -j "2211060112" 
	Sun Nov  6 01:12:00 EST 2022
	(58) horrad at dozer:~/Temp $ TZ=America/Toronto date -j "2211060111" 
	Sun Nov  6 01:11:00 EDT 2022

According to the manpage on OpenBSD a negative tm_isdst means the System 
should try to detect the daylight saving time and there even is a warning,
for no consistent return values in this case.

For reference the manpage:

(56) horrad at dozer:~/Temp $ cat
     mktime() converts the broken-down time, expressed as local time, in the
     structure pointed to by tm into a calendar time value with the same
     encoding as that of the values returned by the time() function.  The
     original values of the tm_wday and tm_yday components of the structure
     are ignored, and the original values of the other components are not
     restricted to their normal ranges.  (A positive or zero value for
     tm_isdst causes mktime() to presume initially that summer time (for
     example, Daylight Saving Time in the U.S.A.) respectively, is or is not
     in effect for the specified time.  A negative value for tm_isdst causes
     the mktime() function to attempt to divine whether summer time is in
     effect for the specified time; in this case it does not use a consistent
     rule and may give a different answer when later presented with the same
     argument.)  On successful completion, the values of the tm_wday and
     tm_yday components of the structure are set appropriately, and the other
     components are set to represent the specified calendar time, but with
     their values forced to their normal ranges; the final value of tm_mday is
     not set until tm_mon and tm_year are determined.  mktime() returns the
     specified calendar time; if the calendar time cannot be represented, it
     returns -1.
     mktime() converts the broken-down time, expressed as local time, in the
     structure pointed to by tm into a calendar time value with the same
     encoding as that of the values returned by the time() function.  The
     original values of the tm_wday and tm_yday components of the structure
     are ignored, and the original values of the other components are not
     restricted to their normal ranges.  (A positive or zero value for
     tm_isdst causes mktime() to presume initially that summer time (for
     example, Daylight Saving Time in the U.S.A.) respectively, is or is not
     in effect for the specified time.  A negative value for tm_isdst causes
     the mktime() function to attempt to divine whether summer time is in
     effect for the specified time; in this case it does not use a consistent
     rule and may give a different answer when later presented with the same
     argument.)  On successful completion, the values of the tm_wday and
     tm_yday components of the structure are set appropriately, and the other
     components are set to represent the specified calendar time, but with
     their values forced to their normal ranges; the final value of tm_mday is
     not set until tm_mon and tm_year are determined.  mktime() returns the
     specified calendar time; if the calendar time cannot be represented, it
     returns -1.