Support OFFSET and (OFFSET ABBR) time zone rules

This simplifies Gnus and VC time zone support, by letting them
feed the output of ‘current-time-zone’ and ‘decode time’ to
primitives that accept time zone arguments.
* doc/lispref/os.texi (Time Zone Rules, Time Conversion):
* etc/NEWS:
* lisp/gnus/message.el (message-insert-formatted-citation-line):
* lisp/org/org.el (org-timestamp-format):
* src/editfns.c (Fformat_time_string, Fdecode_time):
(Fcurrent_time_string, Fcurrent_time_zone, Fset_time_zone_rule):
Document new behavior.
* lisp/gnus/gmm-utils.el (gmm-format-time-string):
* lisp/vc/add-log.el (add-log-iso8601-time-zone):
Mark as obsolete, as it is now just an alias or narrow wrapper
around format-time-string.
* src/editfns.c (tzlookup): Also support integer OFFSET and
list (OFFSET ABBR) as time zone rules.
(Fencode_time): No longer need a special case for a cons ZONE.
(Fcurrent_time_zone): If the time zone string is missing, compute
it the same way the other new code does.

1 files changed, 4 insertions, 15 deletions

diff --git a/lisp/vc/add-log.el b/lisp/vc/add-log.el
index 58a4e77a602..9076d834c7c 100644
--- a/lisp/vc/add-log.el
+++ b/lisp/vc/add-log.el
@@ -590,25 +590,14 @@ If a string, interpret as the ZONE argument of `format-time-string'.")
      (lambda (x) (or (booleanp x) (stringp x))))
 
 (defun add-log-iso8601-time-zone (&optional time zone)
-  (let* ((utc-offset (or (car (current-time-zone time zone)) 0))
-         (sign (if (< utc-offset 0) ?- ?+))
-         (sec (abs utc-offset))
-         (ss (% sec 60))
-         (min (/ sec 60))
-         (mm (% min 60))
-         (hh (/ min 60)))
-    (format (cond ((not (zerop ss)) "%c%02d:%02d:%02d")
-                  ((not (zerop mm)) "%c%02d:%02d")
-                  (t "%c%02d"))
-            sign hh mm ss)))
+  (declare (obsolete nil "25.2"))
+  (format-time-string "%:::z" time zone))
 
 (defvar add-log-iso8601-with-time-zone nil)
 
 (defun add-log-iso8601-time-string (&optional time zone)
-  (let ((date (format-time-string "%Y-%m-%d" time zone)))
-    (if add-log-iso8601-with-time-zone
-        (concat date " " (add-log-iso8601-time-zone time zone))
-      date)))
+  (format-time-string
+   (if add-log-iso8601-with-time-zone "%Y-%m-%d %:::z" "%Y-%m-%d") time zone))
 
 (defun change-log-name ()
   "Return (system-dependent) default name for a change log file."