public function addMonthToBitmask($month)
    {
        $month = (int) $month;
        if ($month < 1 || $month > 12) {
            throw new \RangeException('Month is invalid!');
        }

        $bit     = pow(2, $month - 1);
        $bitmask = $this->getMonthBitmask() | $bit;

        $this->setMonthBitmask($bitmask);

        return $this;
    }