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, 2 deletions

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