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, 77 insertions, 27 deletions

diff --git a/src/editfns.c b/src/editfns.c
index 70285e6d5db..48f2a8de126 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -146,8 +146,6 @@ xtzfree (timezone_t tz)
 static timezone_t
 tzlookup (Lisp_Object zone, bool settz)
 {
-  static char const tzbuf_format[] = "XXX%s%"pI"d:%02d:%02d";
-  char tzbuf[sizeof tzbuf_format + INT_STRLEN_BOUND (EMACS_INT)];
   char const *zone_string;
   timezone_t new_tz;
 
@@ -160,16 +158,53 @@ tzlookup (Lisp_Object zone, bool settz)
     }
   else
     {
+      static char const tzbuf_format[] = "<%+.*"pI"d>%s%"pI"d:%02d:%02d";
+      char const *trailing_tzbuf_format = tzbuf_format + sizeof "<%+.*"pI"d" - 1;
+      char tzbuf[sizeof tzbuf_format + 2 * INT_STRLEN_BOUND (EMACS_INT)];
+      bool plain_integer = INTEGERP (zone);
+
       if (EQ (zone, Qwall))
         zone_string = 0;
       else if (STRINGP (zone))
-        zone_string = SSDATA (zone);
-      else if (INTEGERP (zone))
+        zone_string = SSDATA (ENCODE_SYSTEM (zone));
+      else if (plain_integer || (CONSP (zone) && INTEGERP (XCAR (zone))
+                                 && CONSP (XCDR (zone))))
         {
+          Lisp_Object abbr;
+          if (!plain_integer)
+            {
+              abbr = XCAR (XCDR (zone));
+              zone = XCAR (zone);
+            }
+
           EMACS_INT abszone = eabs (XINT (zone)), hour = abszone / (60 * 60);
-          int min = (abszone / 60) % 60, sec = abszone % 60;
-          sprintf (tzbuf, tzbuf_format, &"-"[XINT (zone) < 0], hour, min, sec);
-          zone_string = tzbuf;
+          int hour_remainder = abszone % (60 * 60);
+          int min = hour_remainder / 60, sec = hour_remainder % 60;
+
+          if (plain_integer)
+            {
+              int prec = 2;
+              EMACS_INT numzone = hour;
+              if (hour_remainder != 0)
+                {
+                  prec += 2, numzone = 100 * numzone + min;
+                  if (sec != 0)
+                    prec += 2, numzone = 100 * numzone + sec;
+                }
+              sprintf (tzbuf, tzbuf_format, prec, numzone,
+                       &"-"[XINT (zone) < 0], hour, min, sec);
+              zone_string = tzbuf;
+            }
+          else
+            {
+              AUTO_STRING (leading, "<");
+              AUTO_STRING_WITH_LEN (trailing, tzbuf,
+                                    sprintf (tzbuf, trailing_tzbuf_format,
+                                             &"-"[XINT (zone) < 0],
+                                             hour, min, sec));
+              zone_string = SSDATA (concat3 (leading, ENCODE_SYSTEM (abbr),
+                                             trailing));
+            }
         }
       else
         xsignal2 (Qerror, build_string ("Invalid time zone specification"),
@@ -1969,9 +2004,13 @@ DEFUN ("format-time-string", Fformat_time_string, Sformat_time_string, 1, 3, 0,
        doc: /* Use FORMAT-STRING to format the time TIME, or now if omitted.
 TIME is specified as (HIGH LOW USEC PSEC), as returned by
 `current-time' or `file-attributes'.  The obsolete form (HIGH . LOW)
-is also still accepted.  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.
+is also still accepted.
+
+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.  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.
 
 The value is a copy of FORMAT-STRING, but with certain constructs replaced
 by text that describes the specified date and time in TIME:
@@ -2085,9 +2124,12 @@ DEFUN ("decode-time", Fdecode_time, Sdecode_time, 0, 2, 0,
 The optional SPECIFIED-TIME should be a list of (HIGH LOW . IGNORED),
 as from `current-time' and `file-attributes', or nil to use the
 current time.  The obsolete form (HIGH . LOW) is also still accepted.
+
 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.
+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.
 
 The list has the following nine members: SEC is an integer between 0
 and 60; SEC is 60 for a leap second, which only some operating systems
@@ -2150,6 +2192,7 @@ check_tm_member (Lisp_Object obj, int offset)
 DEFUN ("encode-time", Fencode_time, Sencode_time, 6, MANY, 0,
        doc: /* Convert SECOND, MINUTE, HOUR, DAY, MONTH, YEAR and ZONE to internal time.
 This is the reverse operation of `decode-time', which see.
+
 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.  It can also be a list (as from
@@ -2184,8 +2227,6 @@ usage: (encode-time SECOND MINUTE HOUR DAY MONTH YEAR &optional ZONE)  */)
   tm.tm_year = check_tm_member (args[5], TM_YEAR_BASE);
   tm.tm_isdst = -1;
 
-  if (CONSP (zone))
-    zone = XCAR (zone);
   timezone_t tz = tzlookup (zone, false);
   value = emacs_mktime_z (tz, &tm);
   xtzfree (tz);
@@ -2214,7 +2255,9 @@ but this is considered obsolete.
 
 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.  */)
+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.  */)
   (Lisp_Object specified_time, Lisp_Object zone)
 {
   time_t value = lisp_seconds_argument (specified_time);
@@ -2290,8 +2333,12 @@ instead of using the current time.  The argument should have the form
 \(HIGH LOW . IGNORED).  Thus, you can use times obtained from
 `current-time' and from `file-attributes'.  SPECIFIED-TIME can also
 have the form (HIGH . LOW), but this is considered obsolete.
-Optional second arg ZONE is omitted or nil for the local time zone, or
-a string as in the TZ environment variable.
+
+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.  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.
 
 Some operating systems cannot provide all this information to Emacs;
 in this case, `current-time-zone' returns a list containing nil for
@@ -2315,15 +2362,18 @@ the data it can't find.  */)
       zone_offset = make_number (offset);
       if (SCHARS (zone_name) == 0)
         {
-          /* No local time zone name is available; use "+-NNNN" instead.  */
-          long int m = offset / 60;
-          long int am = offset < 0 ? - m : m;
-          long int hour = am / 60;
-          int min = am % 60;
-          char buf[sizeof "+00" + INT_STRLEN_BOUND (long int)];
-          zone_name = make_formatted_string (buf, "%c%02ld%02d",
+          /* No local time zone name is available; use numeric zone instead.  */
+          long int hour = offset / 3600;
+          int min_sec = offset % 3600;
+          int amin_sec = min_sec < 0 ? - min_sec : min_sec;
+          int min = amin_sec / 60;
+          int sec = amin_sec % 60;
+          int min_prec = min_sec ? 2 : 0;
+          int sec_prec = sec ? 2 : 0;
+          char buf[sizeof "+0000" + INT_STRLEN_BOUND (long int)];
+          zone_name = make_formatted_string (buf, "%c%.2ld%.*d%.*d",
                                              (offset < 0 ? '-' : '+'),
-                                             hour, min);
+                                             hour, min_prec, min, sec_prec, sec);
         }
     }
 
@@ -2332,11 +2382,11 @@ the data it can't find.  */)
 
 DEFUN ("set-time-zone-rule", Fset_time_zone_rule, Sset_time_zone_rule, 1, 1, 0,
        doc: /* Set the Emacs local time zone using TZ, a string specifying a time zone rule.
-
 If TZ is nil or `wall', use system wall clock time; this differs from
 the usual Emacs convention where nil means current local time.  If TZ
-is t, use Universal Time.  If TZ is an integer, treat it as in
-`encode-time'.
+is t, use Universal Time.  If TZ is a list (as from
+`current-time-zone') or an integer (as from `decode-time'), use the
+specified time zone without consideration for daylight saving time.
 
 Instead of calling this function, you typically want something else.
 To temporarily use a different time zone rule for just one invocation