/**
  * Reset values that shall not be updated by the user directly
  * @param \OCA\Calendar\ICalendar &$new
  * @param \OCA\Calendar\ICalendar $old
  */
 private function resetReadOnlyProperties(ICalendar &$new, ICalendar $old)
 {
     $new->setUserId($old->getUserId());
     $new->setOwnerId($old->getOwnerId());
     $new->setCruds($old->getCruds());
     $new->setCtag($old->getCtag());
 }