protected function isStartTimeValid()
    {
        $this->valid_range_start->setYear(self::$date_year);
        $this->valid_range_start->setMonth(self::$date_month);
        $this->valid_range_start->setDay(self::$date_day);
        $this->valid_range_start->setTZById('UTC');

        return SwatDate::compare(
            $this->value,
            $this->valid_range_start,
            true
        ) >= 0;
    }