[Calendar] Fix wrong start date in corner case

The start date is shifted by a week if the day number of the month's
first day is smaller than the week start's day number. Probably the
only real world examples are months starting on a Sunday with locales
using Monday as start of week.

https://bugzilla.gnome.org/show_bug.cgi?id=625756

js/ui/calendar.js

@@ -226,7 +226,8 @@ Calendar.prototype = {
         iter.setDate(1);
         iter.setSeconds(0);
         iter.setHours(12);
-        iter.setTime(iter.getTime() - (iter.getDay() - this._weekStart) * MSECS_IN_DAY);
+        let daysToWeekStart = (7 + iter.getDay() - this._weekStart) % 7;
+        iter.setTime(iter.getTime() - daysToWeekStart * MSECS_IN_DAY);
 
         let now = new Date();