Fix off-by-one error in decoded-time-add (with months)

* lisp/calendar/time-date.el (decoded-time-add): Fix month
addition, which was off-by-one.

2 files changed, 23 insertions, 3 deletions

diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index 125f9acc705..638d8c1f884 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -401,10 +401,10 @@ changes in daylight saving time are not taken into account."
     (when (decoded-time-year delta)
       (cl-incf (decoded-time-year time) (decoded-time-year delta)))
 
-    ;; Months are pretty simple.
+    ;; Months are pretty simple, but start at 1 (for January).
     (when (decoded-time-month delta)
-      (let ((new (+ (decoded-time-month time) (decoded-time-month delta))))
-        (setf (decoded-time-month time) (mod new 12))
+      (let ((new (+ (1- (decoded-time-month time)) (decoded-time-month delta))))
+        (setf (decoded-time-month time) (1+ (mod new 12)))
         (cl-incf (decoded-time-year time) (/ new 12))))
 
     ;; Adjust for month length (as described in the doc string).
diff --git a/test/lisp/calendar/time-date-tests.el b/test/lisp/calendar/time-date-tests.el
index fe1460cf29e..233d43cd01a 100644
--- a/test/lisp/calendar/time-date-tests.el
+++ b/test/lisp/calendar/time-date-tests.el
@@ -123,4 +123,24 @@
   (should (equal (decoded-time-period '((135 . 10) 0 0 0 0 0 nil nil nil))
                  13.5)))
 
+(ert-deftest test-time-wrap-addition ()
+  (should (equal (decoded-time-add '(0 0 0 1 11 2008 nil nil nil)
+                                   (make-decoded-time :month 1))
+                 '(0 0 0 1 12 2008 nil nil nil)))
+  (should (equal (decoded-time-add '(0 0 0 1 12 2008 nil nil nil)
+                                   (make-decoded-time :month 1))
+                 '(0 0 0 1 1 2009 nil nil nil)))
+  (should (equal (decoded-time-add '(0 0 0 1 11 2008 nil nil nil)
+                                   (make-decoded-time :month 12))
+                 '(0 0 0 1 11 2009 nil nil nil)))
+  (should (equal (decoded-time-add '(0 0 0 1 11 2008 nil nil nil)
+                                   (make-decoded-time :month 13))
+                 '(0 0 0 1 12 2009 nil nil nil)))
+  (should (equal (decoded-time-add '(0 0 0 30 12 2008 nil nil nil)
+                                   (make-decoded-time :day 1))
+                 '(0 0 0 31 12 2008 nil nil nil)))
+  (should (equal (decoded-time-add '(0 0 0 30 12 2008 nil nil nil)
+                                   (make-decoded-time :day 2))
+                 '(0 0 0 1 1 2009 nil nil nil))))
+
 ;;; time-date-tests.el ends here