public function duration()
    {
        $start = $this->startDateTime();
        $end = $this->endDateTime();

        $dateDifference = $start->diff($end);

        $dateInterval = new DateInterval('PT1S');
        $dateInterval->y = $dateDifference->y;
        $dateInterval->m = $dateDifference->m;
        $dateInterval->d = $dateDifference->d;
        $dateInterval->h = $dateDifference->h;
        $dateInterval->i = $dateDifference->i;
        $dateInterval->s = $dateDifference->s;
        $dateInterval->invert = $dateDifference->invert;
        $dateInterval->days = $dateDifference->days;

        return $dateInterval;
    }