function MonthNavigator($url_segment = null, $start_date = null)
 {
     $cal = CalendarUtil::get_calendar_for($url_segment);
     if ($cal) {
         if ($start_date !== null) {
             $start_date = new sfDate(CalendarUtil::getDateFromString($start_date));
         }
         return new MonthNavigator($cal, $start_date);
     }
     return false;
 }