world-clock: Add user option to control order of entries

* lisp/time.el (world-clock-sort-order): Add user option.
(world-clock--sort-entries): Add auxiliary sorting function.
(world-clock-display): Sort entries based on the new user option.
(Bug#78678)

etc/NEWS

@@ -2193,6 +2193,17 @@ Latin-1 range 0-255.  This hard-coded maximum is replaced by
 the display of matching vectors as Unicode strings.  The default value
 is '0xFF' or '255' to preserve the existing behavior.
 
+** Time
+
+*** New user option 'world-clock-sort-order'.
+This option controls the order of timezone entries in the 'world-clock'.
+By default, no sorting is done, and entries appear in the same order as
+in 'world-clock-list'.  Any format understood by 'format-time-string'
+can be used to specify a key for the sort order, which is updated upon
+each refresh.  The sort direction can be controlled by using a cons cell
+of a format string and a boolean.  Alternatively, a sorting function can
+be provided directly.
+
 
 
 * New Modes and Packages in Emacs 31.1

lisp/time.el

@@ -536,6 +536,36 @@ If the value is t instead of an alist, use the value of
   :type 'natnum
   :version "28.1")
 
+(defcustom world-clock-sort-order nil
+  "Sort order of entries in the `world-clock'.
+This variable can take a few different forms:
+
+- A nil value indicates no sorting, and the displayed order is the same
+  as in `world-clock-list'.  This is the default.
+
+- A string is taken as a format for `format-time-string', whose output
+  is used as the sorting key.  For example, an ISO 8601 format `\"%FT%T\"'
+  means entries are sorted in chronological order.
+
+- A cons cell of the form (STRING . BOOL) again provides a format for
+  `format-time-string'.  The boolean part controls the sort direction.
+  For example, `(\"%FT%T\" .  t)' means entries are sorted in reverse
+  chronological order.
+
+- A function that takes a list of (TIMEZONE LABEL) entries and a TIME as
+  arguments, and returns another list of entries."
+  :version "31.1"
+  :type '(choice (const :tag "No sorting" nil)
+		 (const :tag "Chronological order" "%FT%T")
+		 (const :tag "Reverse chronological order" ("%FT%T" . t))
+		 (const :tag "Time of day order" "%T")
+		 (const :tag "Reverse time of day order" ("%T" . t))
+		 (string :tag "Format string")
+		 (cons :tag "Format string and reverse flag"
+		       (string :tag "Format string")
+		       (boolean :tag "Reverse"))
+		 (function :tag "Sorting function")))
+
 (defface world-clock-label
   '((t :inherit font-lock-variable-name-face))
   "Face for time zone label in `world-clock' buffer.")
@@ -562,15 +592,32 @@ See `world-clock'."
 (defvar world-clock--timer nil
   "The current world clock timer.")
 
+(defun world-clock--sort-entries (tzlist time)
+  "Sort TZLIST according to `world-clock-sort-order' at a given TIME."
+  (pcase world-clock-sort-order
+    ((pred null) tzlist)
+    ((or (and (pred stringp) format)
+	 `(,(and (pred stringp) format) .
+	   ,(and (pred booleanp) reverse)))
+     (sort tzlist
+	   :key (lambda (entry)
+		  (format-time-string format time (car entry)))
+	   :reverse reverse))
+    ((pred functionp) (funcall world-clock-sort-order tzlist time))
+    (_ (error "Invalid `world-clock-sort-order': `%s'"
+	      world-clock-sort-order))))
+
 (defun world-clock-display (alist)
-  "Replace current buffer text with times in various zones, based on ALIST."
-  (let ((inhibit-read-only t)
-	(buffer-undo-list t)
-	(now (current-time))
-	(max-width 0)
-	result fmt)
+  "Replace current buffer text with times in various zones, based on ALIST.
+The entries are ordered according to `world-clock-sort-order'."
+  (let* ((inhibit-read-only t)
+	 (buffer-undo-list t)
+	 (now (current-time))
+	 (zones (world-clock--sort-entries alist now))
+	 (max-width 0)
+	 result fmt)
     (erase-buffer)
-    (dolist (zone alist)
+    (dolist (zone zones)
       (let* ((label (cadr zone))
 	     (width (string-width label)))
 	(push (cons label