bug#67706: 30.0.50; timer-next-integral-multiple-of-time does not account for different time-zones

[Eli Zaretskii <eliz@gnu.org>]

> From: Bruno Boal <egomet@bboal.com>
> Cc: 67706@debbugs.gnu.org, info@protesilaos.com
> Date: Fri, 08 Dec 2023 16:51:03 +0000
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Cc: info@protesilaos.com
> >> From: Bruno Boal <egomet@bboal.com>
> >> Date: Fri, 08 Dec 2023 11:51:01 +0000
> >> 
> >> While trying the following snippet in both Lisbon and Athens, we
> >> get the same timer object as showed in list-timers.
> >> 
> >> (run-at-time t 14400 #'message "Testing")
> >> 
> >> What we would expect, is two different timer objects accounting for the
> >> different time zones.
> >> 
> >> We did a edebug and found out that the function aforementioned on the
> >> subject is always returning the same value despite of the different
> >> local times.
> >> 
> >> Are we missing something obvious or is this a bug?
> >
> > Please show a minimal recipe, starting from "emacs -Q", to reproduce
> > the issue you are seeing.  I'm not sure I understand all the details,
> > and therefore don't follow why you expected two objects.
> 
> Let me try to demonstrate the possible issue.
> 
> Running the following snippet in my PC, with Lisbon time of 16h36:
> 
>      (run-at-time t 14400 #'message "Testing")
> 
> Checking result of `list-timers' function:
> 
> Next           Repeat      Function
> 3h 24m 34.7s       4h      message
> 
> Running the same snippet in Prot's PC, with Athens time of 18h36:
> 
>      (run-at-time t 14400 #'message "Testing")
> 
> Checking result of `list-timers' function:
> 
> Next           Repeat      Function
> 3h 24m 34.7s       4h      message
> 
> So despite the difference of time-zones the next occurrence of Function
> has the same Next time interval. Is this the expected behavior? Because
> reading the documentation I would expect the Function to have a Next
> interval multiple of Repeat in order to run at 20h local time of the
> machine where the code was evaluated. Being this the case, the Next
> value in Prot's PC would have to be 1h 24m34.7s.

Sorry, I still don't follow: these are two separate systems set up
with two different time zones, is that right?  If so, why is it
surprising that each system produces the same result in list-timers?
The argument 14400 means "14400 seconds from now", and is independent
of the time zone of the machine, since it's a relative time.