bug#72570: 31.0.50; Regression in date-to-time

[Ulrich Mueller <ulm@gentoo.org>]

>>>>> On Tue, 13 Aug 2024, Paul Eggert wrote:

> As for the original issue, I tend to agree with Ulrich that
> date-to-time should default to local time. This would need changes to
> both documentation and to code. Ulrich mentioned the following
> potential issue with this idea:

>> The problem is that timezone-make-date-arpa-standard would need an
>> explicit timezone as its second argument, which we don't know.

> The code could use timezone-time-zone-from-absolute to infer a
> timezone. timezone-fix-time already does this sort of thing.
> Admittedly getting the details right could be a bit tricky, and there
> is no perfect solution in this area (certainly timezone-fix-time is
> flawed). It might be good enough, though.

IIUC timezone-time-zone-from-absolute expects the SECONDS argument
to be seconds since midnight in UTC (again, not documented). In the
Europe/Berlin timezone (DST starts at 02:00 local time), I get this:

  (timezone-time-zone-from-absolute
   (timezone-absolute-from-gregorian 3 31 2024)
   (* 1 60 60))

  (7200 "CEST")

So don't we have a chicken-and-egg problem? We'd need the UTC timestamp
to infer the timezone, but the timezone to calculate the timestamp?