Index: public/javascripts/calendar/calendar.js
===================================================================
--- public/javascripts/calendar/calendar.js	(revision 3810)
+++ public/javascripts/calendar/calendar.js	(working copy)
@@ -1105,7 +1105,7 @@
 	var day1 = (date.getDay() - this.firstDayOfWeek) % 7;
 	if (day1 < 0)
 		day1 += 7;
-	date.setDate(-day1);
+	date.setDate(0-day1);
 	date.setDate(date.getDate() + 1);
 
 	var row = this.tbody.firstChild;