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.

4 files changed, 17 insertions, 50 deletions

diff --git a/lisp/gnus/gmm-utils.el b/lisp/gnus/gmm-utils.el
index f6455cf9f1a..7aa52794e4c 100644
--- a/lisp/gnus/gmm-utils.el
+++ b/lisp/gnus/gmm-utils.el
@@ -256,37 +256,8 @@ If mode is nil, use `major-mode' of the current buffer."
                  (string-match "^\\(.+\\)-mode$" mode)
                  (match-string 1 mode))))))
 
-(defun gmm-format-time-string (format-string &optional time tz)
-  "Use FORMAT-STRING to format the time TIME, or now if omitted.
-The optional TZ specifies the time zone in a number of seconds; any
-other non-nil value will be treated as 0.  Note that both the format
-specifiers `%Z' and `%z' will be replaced with a numeric form. "
-;; FIXME: is there a smart way to replace %Z with a time zone name?
-  (if (and (numberp tz) (not (zerop tz)))
-      (let ((st 0)
-            (case-fold-search t)
-            ls nd rest)
-        (setq time (if time
-                       (copy-sequence time)
-                     (current-time)))
-        (if (>= (setq ls (- (cadr time) (car (current-time-zone)) (- tz))) 0)
-            (setcar (cdr time) ls)
-          (setcar (cdr time) (+ ls 65536))
-          (setcar time (1- (car time))))
-        (setq tz (format "%s%02d%02d"
-                         (if (>= tz 0) "+" "-")
-                         (/ (abs tz) 3600)
-                         (/ (% (abs tz) 3600) 60)))
-        (while (string-match "%+z" format-string st)
-          (if (zerop (% (- (setq nd (match-end 0)) (match-beginning 0)) 2))
-              (progn
-                (push (substring format-string st (- nd 2)) rest)
-                (push tz rest))
-            (push (substring format-string st nd) rest))
-          (setq st nd))
-        (push (substring format-string st) rest)
-        (format-time-string (apply 'concat (nreverse rest)) time))
-    (format-time-string format-string time t)))
+(define-obsolete-function-alias 'gmm-format-time-string 'format-time-string
+  "25.2")
 
 (provide 'gmm-utils)
 
diff --git a/lisp/gnus/message.el b/lisp/gnus/message.el
index 14d8d30f8af..32d740b0190 100644
--- a/lisp/gnus/message.el
+++ b/lisp/gnus/message.el
@@ -3879,8 +3879,13 @@ This function uses `mail-citation-hook' if that is non-nil."
 (defun message-insert-formatted-citation-line (&optional from date tz)
   "Function that inserts a formatted citation line.
 The optional FROM, and DATE are strings containing the contents of
-the From header and the Date header respectively.  The optional TZ
-is a number of seconds, overrides the time zone of DATE.
+the From header and the Date header respectively.
+
+The optional TZ is omitted or nil for Emacs local time, t for
+Universal Time, `wall' for system wall clock time, or a string as
+in the TZ environment variable.  It can also be a list (as from
+`current-time-zone') or an integer (as from `decode-time')
+applied without consideration for daylight saving time.
 
 See `message-citation-line-format'."
   ;; The optional args are for testing/debugging.  They will disappear later.
diff --git a/lisp/org/org.el b/lisp/org/org.el
index 231daa9a6a7..3abf62704bb 100644
--- a/lisp/org/org.el
+++ b/lisp/org/org.el
@@ -22673,8 +22673,10 @@ When optional argument END is non-nil, use end of date-range or
 time-range, if possible.
 
 The optional ZONE is omitted or nil for Emacs local time, t for
-Universal Time, `wall' for system wall clock time, or a string as in
-the TZ environment variable."
+Universal Time, `wall' for system wall clock time, or a string as
+in the TZ environment variable.  It can also be a list (as from
+`current-time-zone') or an integer (as from `decode-time')
+applied without consideration for daylight saving time."
   (format-time-string
    format
    (apply 'encode-time
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."