public function numberOfDays()
    {
        if (self::APRIL === $this->month
            || self::JUNE === $this->month
            || self::SEPTEMBER === $this->month
            || self::NOVEMBER === $this->month
        ) {
            return 30;
        }

        if (self::FEBRURY === $this->month) {
            return $this->year->isLeap() ? 29 : 28;
        }

        return 31;
    }