master cb9fc5e: Fix off-by-one error in decoded-time-add (with months)

[Lars Ingebrigtsen]

branch: master
commit cb9fc5e7731e506e4e0facd3d060d19e388b32ac
Author: Lars Ingebrigtsen <larsi@gnus.org>
Commit: Lars Ingebrigtsen <larsi@gnus.org>

    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.
---
 lisp/calendar/time-date.el            |  6 +++---
 test/lisp/calendar/time-date-tests.el | 20 ++++++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index 125f9ac..638d8c1 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 fe1460c..233d43c 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