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

[Ulrich Mueller <ulm@gentoo.org>]

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Attached patch will make date-to-time work as documented.

Alternatively, maybe it would be more consistent with other time
functions in Emacs if date-to-time would use local time as the default
(i.e. what is assumed by the current unit test in time-date-tests.el).

However, the fallback code uses Universal Time, and I don't see an easy
way to modify it for local time. The problem is that
timezone-make-date-arpa-standard would need an explicit timezone as its
second argument, which we don't know. We cannot use (current-time-zone)
there, because it's the timezone of the current time, not the one of the
to-be-converted timestamp. (For example, in the Europe/Berlin timezone,
(current-time-zone) returns (7200 "CEST"), while for conversion of
"2024-01-01 00:00:00" one would need (3600 "CET")).

(BTW, the code in timezone.el is very brittle. For example,
(timezone-make-date-arpa-standard "2024-01-01") signals a
wrong-type-argument error from 'capitalize'.)


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From 6c55891b5850c923196187b21b73f52d474c325a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ulrich=20M=C3=BCller?= <ulm@gentoo.org>
Date: Mon, 12 Aug 2024 07:30:01 +0200
Subject: [PATCH] Make date-to-time without timezone work as documented

* lisp/calendar/time-date.el (date-to-time): Assume Universal Time
rather than local time when no timezone is specified.  (Bug#72570)
* test/lisp/calendar/time-date-tests.el (test-date-to-time):
Update test.
---
 lisp/calendar/time-date.el            | 5 +++--
 test/lisp/calendar/time-date-tests.el | 8 ++++++--
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index eca80f1e8b6..2f3ef0d8b4c 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -156,7 +156,7 @@ date-to-time
   (condition-case err
       (let ((parsed (parse-time-string date)))
 	(when (decoded-time-year parsed)
-	  (decoded-time-set-defaults parsed))
+	  (decoded-time-set-defaults parsed t))
 	(encode-time parsed))
     (error
      (let ((overflow-error '(error "Specified time is not representable")))
@@ -164,7 +164,8 @@ date-to-time
 	   (signal (car err) (cdr err))
 	 (condition-case err
 	     (encode-time (parse-time-string
-			   (timezone-make-date-arpa-standard date)))
+			   (timezone-make-date-arpa-standard
+			    date nil '(0 "UTC"))))
 	   (error
 	    (if (equal err overflow-error)
 		(signal (car err) (cdr err))
diff --git a/test/lisp/calendar/time-date-tests.el b/test/lisp/calendar/time-date-tests.el
index 6512dd0bd07..d7d39f050b7 100644
--- a/test/lisp/calendar/time-date-tests.el
+++ b/test/lisp/calendar/time-date-tests.el
@@ -42,8 +42,12 @@ test-obsolete-encode-time-value
                  '(1 2 3 4))))
 
 (ert-deftest test-date-to-time ()
-  (should (equal (format-time-string "%F %T" (date-to-time "2021-12-04"))
-                 "2021-12-04 00:00:00")))
+  (should (equal (format-time-string
+                  "%F %T" (date-to-time "2021-12-04") t)
+                 "2021-12-04 00:00:00"))
+  (should (equal (format-time-string
+                  "%F %T" (date-to-time "2021-12-04 00:00:00 +0100") t)
+                 "2021-12-03 23:00:00")))
 
 (ert-deftest test-days-between ()
   (should (equal (days-between "2021-10-22" "2020-09-29") 388)))
-- 
2.45.2