diff --git a/addons/web_calendar/static/src/js/web_calendar.js b/addons/web_calendar/static/src/js/web_calendar.js
index 282375f1c1b..fbf051973b9 100644
--- a/addons/web_calendar/static/src/js/web_calendar.js
+++ b/addons/web_calendar/static/src/js/web_calendar.js
@@ -654,7 +654,7 @@ openerp.web_calendar = function(instance) {
             }
 
             if (this.date_delay) {
-                var diff_seconds = Math.round((event_end.getTime() - event.start.getTime()) / 1000);
+                var diff_seconds = Math.round((date_stop_day.getTime() - date_start_day.getTime()) / 1000);
                 data[this.date_delay] = diff_seconds / 3600;
             }
             return data;