format-time-string subsumes time-zone-format

* lisp/calendar/time-date.el (time-zone-format):
* test/lisp/calendar/time-date-tests.el (test-time-zone-format):
Remove.
* lisp/gnus/nnrss.el (nnrss-normalize-date):
Use format-time-string instead of time-zone-format.

etc/NEWS

@@ -2110,10 +2110,6 @@ doing computations on a decoded time structure), 'make-decoded-time'
 filled out), and 'encoded-time-set-defaults' (which fills in nil
 elements as if it's midnight January 1st, 1970) have been added.
 
-*** The new function `time-zone-format' has been added to format
-Emacs time zones (which are in seconds) according to many standards
-(i.e., "+01:00").
-
 ** 'define-minor-mode' automatically documents the meaning of ARG.
 
 +++

lisp/calendar/time-date.el

@@ -352,20 +352,6 @@ is output until the first non-zero unit is encountered."
                          (<= (car here) delay)))
              (concat (format "%.2f" (/ delay (car (cddr here)))) (cadr here))))))
 
-(defun time-zone-format (seconds &optional short)
-  "Format SECONDS as a valid time zone string.
-For instance, 3600 is \"+01:00\".
-If SHORT, the colon isn't included."
-  (format "%s%02d%s%02d"
-          (if (< seconds 0)
-              "-"
-            "+")
-          (/ (abs seconds) 3600)
-          (if short
-              ""
-            ":")
-          (mod (abs seconds) 3600)))
-
 (defun date-days-in-month (year month)
   "The number of days in MONTH in YEAR."
   (if (= month 2)

lisp/gnus/nnrss.el

@@ -487,7 +487,7 @@ which RSS 2.0 allows."
 	  (format "%s, %02d %s %04d %s%s"
 		  (substring cts 0 3) day (substring cts 4 7) year time
 		  (if zone
-		      (concat " " (time-zone-format zone t))
+		      (concat " " (format-time-string "%z" nil zone))
 		    "")))
       (message-make-date given))))
 

test/lisp/calendar/time-date-tests.el

@@ -104,12 +104,6 @@
     (should (equal (decoded-time-add time (mdec :zone -7200))
                    '(12 15 14 8 7 2019 1 t 7200)))))
 
-(ert-deftest test-time-zone-format ()
-  (should (equal (time-zone-format 3600)
-                 "+01:00"))
-  (should (equal (time-zone-format -7200)
-                 "-02:00")))
-
 (require 'ert)
 
 ;;; time-date-tests.el ends here