public function offsetDatetime($time = null, $offset = 0)
    {
        if ($this->timeFormat === static::$timeFormatDatetime) {
            return date('Y-m-d H:i:s', strtotime(($offset >= 0 ? "+$offset" : $offset) . " seconds", is_string($time) ? strtotime($time) : (is_int($time) ? $time : time())));
        }
        if ($this->timeFormat === static::$timeFormatTimestamp) {
            return (is_int($time) ? $time : time()) + $offset;
        }
        return null;
    }