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, 6 insertions, 5 deletions

diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
index 6e0edec2943..becb691581b 100644
--- a/doc/lispref/os.texi
+++ b/doc/lispref/os.texi
@@ -1325,7 +1325,12 @@ omitted or @code{nil}, the conversion uses Emacs's default time zone.
 If it is @code{t}, the conversion uses Universal Time.  If it is
 @code{wall}, the conversion uses the system wall clock time.  If it is
 a string, the conversion uses the time zone rule equivalent to setting
-@env{TZ} to that string.
+@env{TZ} to that string.  If it is an integer @var{offset}, the
+conversion uses a fixed time zone with the given offset and a numeric
+abbreviation.  If it is a list (@var{offset} @var{abbr}), where
+@var{offset} is an integer number of seconds east of Universal Time
+and @var{abbr} is a string, the conversion uses a fixed time zone with
+the given offset and abbreviation.
 
 @defun current-time-zone &optional time zone
 @cindex time zone, current
@@ -1423,10 +1428,6 @@ yourself before you call @code{encode-time}.
 
 The optional argument @var{zone} defaults to the current time zone rule.
 @xref{Time Zone Rules}.
-In addition to the usual time zone rule values, it can also be a list
-(as you would get from @code{current-time-zone}) or an integer (as
-from @code{decode-time}), applied without any further alteration for
-daylight saving time.
 
 If you pass more than seven arguments to @code{encode-time}, the first
 six are used as @var{seconds} through @var{year}, the last argument is