public function setStartTimeOfDay(\DateTime $startTimeOfDay)
    {
        $startTimeOfDay = clone $startTimeOfDay;
        $startTimeOfDay->setDate(1970, 1, 1); // zero date

        if ($this->getEndTimeOfDay() && $this->getEndTimeOfDay() < $startTimeOfDay) {
            throw new \InvalidArgumentException('End time of day cannot be before start time of day');
        }

        $this->setValue('startTimeOfDay', $startTimeOfDay);
    }