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, 8 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 00f5aadd856..5ebff6267fe 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -228,6 +228,14 @@ two objects are 'eq' ('eql'), then the result of 'sxhash-eq'
 consistency with the new functions.  For compatibility, 'sxhash'
 remains as an alias to 'sxhash-equal'.
 
++++
+** Time conversion functions that accept a time zone rule argument now
+allow it to be OFFSET or a list (OFFSET ABBR), where the integer
+OFFSET is a count of seconds east of Universal Time, and the string
+ABBR is a time zone abbreviation.  The affected functions are
+'current-time-string', 'current-time-zone', 'decode-time',
+'format-time-string', and 'set-time-zone-rule'.
+
 
 * Changes in Emacs 25.2 on Non-Free Operating Systems