bug#55635: `make-decoded-time' incorrectly sets DST to nil, it should be -1 (guess)

[Lars Ingebrigtsen <larsi@gnus.org>]

Maxim Nikulin <m.a.nikulin@gmail.com> writes:

> Consider the following example:
>
>     (format-time-string
>      "%F %T %Z %z"
>      (encode-time
>       (make-decoded-time :year 2022 :month 3 :day 31
>     			 :hour 23 :minute 30 :second 0
>     			 :zone "Europe/Madrid"))
>      "Europe/Madrid")
>     "2022-04-01 00:30:00 CEST +0200"
>
> I believe that the result should be
>     "2022-03-31 23:30:00 CEST +0200"
> It can be obtained if :dst -1 is explicitly added to the
> `make-decoded-time' arguments.
>
> Since `make-decoded-time' is defined using `cl-defun', I think, it is
> better to use -1 ("guess") default value for the :dst argument, not
> nil that explicitly says "no daylight saving time".

I think that makes some sense, but on the other hand, that's just a
simple helper function that does what it says -- "with only the keywords
given filled out".  But perhaps -1 is less "filled out" than nil in this
case.

> There is `decoded-time-set-defaults', but it does not help
>
>     (format-time-string
>      "%F %T %Z %z"
>      (encode-time
>       (decoded-time-set-defaults
>        (make-decoded-time :year 2022 :month 3 :day 31
> 			  :hour 23 :minute 30)
>        "Europe/Madrid"))
>      "Europe/Madrid")
>     "2022-04-01 01:30:00 CEST +0200"
>
> This case I have no idea how to fix the issue.

It's this code, I guess:

  ;; When we don't have a time zone, default to DEFAULT-ZONE without
  ;; DST if DEFAULT-ZONE if given, and to unknown DST otherwise.
  (unless (decoded-time-zone time)
    (if default-zone
	(progn (setf (decoded-time-zone time) default-zone)
	       (setf (decoded-time-dst time) nil))
      (setf (decoded-time-dst time) -1)))

I've added Paul to the CCs; perhaps he has some comments.

-- 
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   bloggy blog: http://lars.ingebrigtsen.no