Get dropoff date from delivery date

    const WEEKDAY_MONDAY = 1;

    /**
     * Get drop off date
     *
     * @param int $dateTime
     * @return int
     */
    public function getDropOffDateTimeFromDeliveryDateTime($dateTime)
    {
        $weekDay = date('N', $dateTime);

        if ($weekDay === self::WEEKDAY_MONDAY) {
            return strtotime("-2 day", $dateTime);
        }

        return strtotime("-1 day", $dateTime);
    }

Make sure the day is in the future:

        if (new \DateTime() > new \DateTime($delivery_date)) {
            $datetime = new \DateTime();
            $datetime->modify('+1 day');
            $delivery_date = $datetime->format('Y\-m\-d\ h:i:s');
        }