/**
  * Checks id the dates for the calendar are valid
  *
  * @param $start Date string in this format: Y-m-d i.e. 2015-06-22
  * @param $end Date string in this format: Y-m-d i.e. 2015-06-22
  */
 public static function check_if_valid_start_end_date_calendar_or_die($start, $end)
 {
     // Check if start date is valid
     $start_parsed = Resource_Booking_Ajax_Common::check_if_valid_date_or_die($start);
     // Check if end date is valid
     $end_parsed = Resource_Booking_Ajax_Common::check_if_valid_date_or_die($end);
     // Check if start < end
     Resource_Booking_Ajax_Common::check_if_start_time_before_end_or_die($start_parsed, $end_parsed);
 }