protected function isInitDatetime($attribute)
    {
        if ($this->timeFormat === static::$timeFormatDatetime) {
            return $attribute == static::$initDatetime || $attribute == null;
        }
        if ($this->timeFormat === static::$timeFormatTimestamp) {
            return $attribute == static::$initTimestamp || $attribute == null;
        }
        return false;
    }