Drop fallback code in date-to-time, update documentation

* lisp/calendar/time-date.el (date-to-time): Drop fallback code.
Document that the default timezone is local time, rather than GMT.
* test/lisp/calendar/time-date-tests.el (test-date-to-time):
Add more test cases.
* doc/lispref/os.texi (Time Parsing): Document that 'date-to-time'
defaults to local time.
* etc/NEWS: Announce the change.  (Bug#72570)

4 files changed, 37 insertions, 19 deletions

diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 5839de4a650..ddaa1de221c 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1788,8 +1788,8 @@ Conversion}).
 This function parses the time-string @var{string} and returns the
 corresponding Lisp timestamp.  The argument @var{string} should represent
 a date-time, and should be in one of the forms recognized by
-@code{parse-time-string} (see below).  This function assumes Universal
-Time if @var{string} lacks explicit time zone information,
+@code{parse-time-string} (see below).  This function assumes local time
+if @var{string} lacks explicit time zone information,
 and assumes earliest values if @var{string} lacks month, day, or time.
 The operating system limits the range of time and zone values.
 @end defun
diff --git a/etc/NEWS b/etc/NEWS
index b89a80aa14d..0db5c74956d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -257,6 +257,15 @@ language A.
 If supplied, 'string-pixel-width' will use any face remappings from
 BUFFER when computing the string's width.
 
++++
+** 'date-to-time' now defaults to local time
+The function now assumes local time instead of Universal Time when
+its argument lacks explicit time zone information.  This has been the
+de-facto behavior since Emacs 24 although documentation said otherwise.
+Also, the fallback on 'timezone-make-date-arpa-standard' has been
+removed because its supported date styles can be handled by
+'parse-time-string'.
+
 
 * Changes in Emacs 31.1 on Non-Free Operating Systems
 
diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index b441974b943..9a2fb45e3bc 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -145,14 +145,10 @@ it is assumed that PICO was omitted and should be treated as zero."
 (autoload 'timezone-make-date-arpa-standard "timezone")
 
 ;;;###autoload
-;; `parse-time-string' isn't sufficiently general or robust.  It fails
-;; to grok some of the formats that timezone does (e.g. dodgy
-;; post-2000 stuff from some Elms) and either fails or returns bogus
-;; values.  timezone-make-date-arpa-standard should help.
 (defun date-to-time (date)
   "Parse a string DATE that represents a date-time and return a time value.
 DATE should be in one of the forms recognized by `parse-time-string'.
-If DATE lacks timezone information, GMT is assumed."
+If DATE lacks time zone information, local time is assumed."
   (condition-case err
       ;; Parse DATE. If it contains a year, use defaults for other components.
       ;; Then encode the result; this signals an error if the year is missing,
@@ -164,16 +160,9 @@ If DATE lacks timezone information, GMT is assumed."
           (decoded-time-set-defaults parsed))
         (encode-time parsed))
     (error
-     (let ((overflow-error '(error "Specified time is not representable")))
-       (if (equal err overflow-error)
-           (signal (car err) (cdr err))
-         (condition-case err
-             (encode-time (parse-time-string
-                           (timezone-make-date-arpa-standard date)))
-           (error
-            (if (equal err overflow-error)
-                (signal (car err) (cdr err))
-              (error "Invalid date: %s" date)))))))))
+     (if (equal err '(error "Specified time is not representable"))
+         (signal (car err) (cdr err))
+       (error "Invalid date: %s" date)))))
 
 ;;;###autoload
 (defalias 'time-to-seconds #'float-time)
diff --git a/test/lisp/calendar/time-date-tests.el b/test/lisp/calendar/time-date-tests.el
index 6512dd0bd07..f8e434e17b1 100644
--- a/test/lisp/calendar/time-date-tests.el
+++ b/test/lisp/calendar/time-date-tests.el
@@ -42,8 +42,28 @@
                  '(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")))
+  (let ((date-list
+         '(("2021-12-04"                   (00 00 00 04 12 2021 nil -1 nil))
+           ("2006-05-04T03:02:01Z"         (01 02 03 04 05 2006 nil nil 0))
+           ;; Test cases from timezone-parse-date docstring
+           ("14 Apr 89 03:20"              (00 20 03 14 04 1989 nil -1 nil))
+           ("14 Apr 89 03:20:12 GMT"       (12 20 03 14 04 1989 nil nil 0))
+           ("Fri, 17 Mar 89 4:01"          (00 01 04 17 03 1989 nil -1 nil))
+           ("Fri, 17 Mar 89 4:01:33 GMT"   (33 01 04 17 03 1989 nil nil 0))
+           ("Mon Jan 16 16:12 1989"        (00 12 16 16 01 1989 nil -1 nil))
+           ("Mon Jan 16 16:12:37 GMT 1989" (37 12 16 16 01 1989 nil nil 0))
+           ("Thu, 11 Apr 16:17:12 91"      (12 17 16 11 04 1991 nil -1 nil))
+           ("Mon, 6  Jul 16:47:20 T 1992"  (20 47 16 06 07 1992 nil -1 nil))
+           ("1996-06-24 21:13:12"          (12 13 21 24 06 1996 nil -1 nil))
+           ("19960624t211312"              (12 13 21 24 06 1996 nil -1 nil))
+           ;; These are parsed incorrectly:
+           ;; "6 May 1992 1641-JST (Wednesday)"
+           ;; "22-AUG-1993 10:59:12.82"
+           ;; "1996-06-24 21:13-ZONE"
+           )))
+    (dolist (date date-list)
+      (should (equal (date-to-time (car date))
+                     (encode-time (cadr date)))))))
 
 (ert-deftest test-days-between ()
   (should (equal (days-between "2021-10-22" "2020-09-29") 388)))