private function intervalToSeconds(DateInterval $interval): int
    {
        $seconds = (int) $interval->format('%s');

        $multiplier = 60;
        $seconds += (int) $interval->format('%i') * $multiplier;

        $multiplier *= 60;
        $seconds += (int) $interval->format('%h') * $multiplier;

        $multiplier *= 24;
        $seconds += (int) $interval->format('%d') * $multiplier;

        $multiplier *= 30;
        $seconds += (int) $interval->format('%m') * $multiplier;

        $multiplier *= 12;
        $seconds += (int) $interval->format('%y') * $multiplier;

        return $seconds;
    }