diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index 01f96305edb..9041b04ae62 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -547,7 +547,7 @@ changes in daylight saving time are not taken into account."
     (when (decoded-time-month delta)
       (let ((new (+ (1- (decoded-time-month time)) (decoded-time-month delta))))
         (setf (decoded-time-month time) (1+ (mod new 12)))
-        (incf (decoded-time-year time) (- (/ new 12) (if (< new 0) 1 0)))))
+        (incf (decoded-time-year time) (floor new 12))))
 
     ;; Adjust for month length (as described in the doc string).
     (setf (decoded-time-day time)