time-stamp-time-zone: update customization

* time-stamp.el (time-stamp-time-zone): Support customization with
an integer offset (a new possible value of the ZONE argument to
format-time-string in Emacs 27).
Update the safe-local-variable predicate from string-or-null-p
(describing time-stamp-time-zone's domain before 2015) to new
predicate time-stamp-zone-type-p (describing the current domain).

* time-stamp-tests.el (time-stamp-test-helper-zone-type-p): New test.

1 files changed, 18 insertions, 2 deletions

diff --git a/lisp/time-stamp.el b/lisp/time-stamp.el
index 094ef915265..6b1ff3e618c 100644
--- a/lisp/time-stamp.el
+++ b/lisp/time-stamp.el
@@ -109,10 +109,26 @@ Its format is that of the ZONE argument of the `format-time-string' function."
   :type '(choice (const :tag "Emacs local time" nil)
                  (const :tag "Universal Time" t)
                  (const :tag "system wall clock time" wall)
-                 (string :tag "TZ environment variable value"))
+                 (string :tag "TZ environment variable value")
+                 (list :tag "Offset and name"
+                       (integer :tag "Offset (seconds east of UTC)")
+                       (string :tag "Time zone abbreviation"))
+                 (integer :tag "Offset (seconds east of UTC)"))
   :group 'time-stamp
   :version "20.1")
-;;;###autoload(put 'time-stamp-time-zone 'safe-local-variable 'string-or-null-p)
+;;;###autoload(put 'time-stamp-time-zone 'safe-local-variable 'time-stamp-zone-type-p)
+
+;;;###autoload
+(defun time-stamp-zone-type-p (zone)
+  "Return whether or not ZONE is of the correct type for a timezone rule.
+Valid ZONE values are described in the documentation of `format-time-string'."
+  (or (memq zone '(nil t wall))
+      (stringp zone)
+      (and (consp zone)
+           (integerp (car zone))
+           (consp (cdr zone))
+           (stringp (cadr zone)))
+      (integerp zone)))
 
 ;;; Do not change time-stamp-line-limit, time-stamp-start,
 ;;; time-stamp-end, time-stamp-pattern, time-stamp-inserts-lines,