Add a variable to allow displaying numeric time zones

* lisp/calendar/calendar.el (calendar-use-numeric-time-zones): New
variable.

* doc/emacs/calendar.texi (Sunrise/Sunset): Document it (bug#33149).

* lisp/calendar/cal-dst.el (calendar-standard-time-zone-name): Use it.
(calendar-daylight-time-zone-name): Ditto.

* lisp/calendar/solar.el (sunrise-sunset): Adjust usage.
(solar-equinoxes-solstices): Ditto.

lisp/calendar/cal-dst.el

@@ -350,17 +350,29 @@ If the locale never uses daylight saving time, set this to 0."
   :group 'calendar-dst)
 
 (defcustom calendar-standard-time-zone-name
-  (or (nth 2 calendar-current-time-zone-cache) "EST")
+  (if calendar-use-numeric-time-zones
+      (if calendar-current-time-zone-cache
+          (format-time-string
+           "%z" 0 (* 60 (car calendar-current-time-zone-cache)))
+        "+0000")
+    (or (nth 2 calendar-current-time-zone-cache) "EST"))
   "Abbreviated name of standard time zone at `calendar-location-name'.
 For example, \"EST\" in New York City, \"PST\" for Los Angeles."
   :type 'string
+  :version "28.1"
   :group 'calendar-dst)
 
 (defcustom calendar-daylight-time-zone-name
-  (or (nth 3 calendar-current-time-zone-cache) "EDT")
+  (if calendar-use-numeric-time-zones
+      (if calendar-current-time-zone-cache
+          (format-time-string
+           "%z" 0 (* 60 (cadr calendar-current-time-zone-cache)))
+        "+0000")
+    (or (nth 3 calendar-current-time-zone-cache) "EDT"))
   "Abbreviated name of daylight saving time zone at `calendar-location-name'.
 For example, \"EDT\" in New York City, \"PDT\" for Los Angeles."
   :type 'string
+  :version "28.1"
   :group 'calendar-dst)
 
 (defcustom calendar-daylight-savings-starts-time

lisp/calendar/calendar.el

@@ -1061,6 +1061,13 @@ calendar."
   :type 'boolean
   :group 'holidays)
 
+(defcustom calendar-use-numeric-time-zones nil
+  "If nil, use symbolic time zones like \"CET\" when displaying dates.
+If non-nil, use numeric time zines like \"+0100\"."
+  :type 'boolean
+  :version "28.1"
+  :group 'calendar)
+
 ;;; End of user options.
 
 (calendar-recompute-layout-variables)

lisp/calendar/solar.el

@@ -840,7 +840,9 @@ This function is suitable for execution in an init file."
                             "E" "W"))))))
          (calendar-standard-time-zone-name
           (if (< arg 16) calendar-standard-time-zone-name
-            (cond ((zerop calendar-time-zone) "UTC")
+            (cond ((zerop calendar-time-zone)
+                   (if calendar-use-numeric-time-zones
+                       "+0100" "UTC"))
                   ((< calendar-time-zone 0)
                    (format "UTC%dmin" calendar-time-zone))
                   (t  (format "UTC+%dmin" calendar-time-zone)))))
@@ -1013,7 +1015,10 @@ Requires floating point."
   (let* ((m displayed-month)
          (y displayed-year)
          (calendar-standard-time-zone-name
-          (if calendar-time-zone calendar-standard-time-zone-name "UTC"))
+          (cond
+           (calendar-time-zone calendar-standard-time-zone-name)
+           (calendar-use-numeric-time-zones "+0100")
+           (t "UTC")))
          (calendar-daylight-savings-starts
           (if calendar-time-zone calendar-daylight-savings-starts))
          (calendar-daylight-savings-ends