public static function difference($startTime, $endTime = null, array $options = []) {
		if (!is_int($startTime)) {
			$startTime = strtotime($startTime);
		}
		if (!is_int($endTime)) {
			$endTime = strtotime($endTime);
		}
		//@FIXME: make it work for > month
		return abs($endTime - $startTime);
	}