public function calculateDays(string $deliveryDate, string $referenceDate): int
    {
        $timestampDeliveryDate = $this->DM->convertDateToTimestamp($deliveryDate);
        $timestampReferenceDate = $this->DM->convertDateToTimestamp($referenceDate);
        $timestampConceptionDate = $timestampDeliveryDate - $this->pregnancyDuration;
        return ceil(($timestampReferenceDate - $timestampConceptionDate) / $this->periodDay);
    }