Allow adding time zones

lisp/calendar/time-date.el

@@ -421,6 +421,10 @@ changes in daylight saving time are not taken into account."
     (setq seconds (+ (* (or (decoded-time-hour delta) 0) 3600)
                      (* (or (decoded-time-minute delta) 0) 60)
                      (or (decoded-time-second delta) 0)))
+
+    ;; Time zone adjustments are basically the same as time adjustments.
+    (setq seconds (+ seconds (or (decoded-time-zone delta) 0)))
+
     (cond
      ((> seconds 0)
       (decoded-time--alter-second time seconds t))

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

@@ -97,6 +97,13 @@
                    '(12 15 16 9 7 2019 1 t 7200)))
     ))
 
+(ert-deftest test-decoded-add-zone ()
+  (let ((time '(12 15 16 8 7 2019 1 t 7200)))
+    (should (equal (decoded-time-add time (mdec :zone -3600))
+                   '(12 15 15 8 7 2019 1 t 7200)))
+    (should (equal (decoded-time-add time (mdec :zone -7200))
+                   '(12 15 14 8 7 2019 1 t 7200)))))
+
 (require 'ert)
 
 ;;; time-date-tests.el ends here