private function isValidMonth($month)
    {
        $list = array(
            self::MONTH_JANUARY,
            self::MONTH_FEBRUARY,
            self::MONTH_MARCH,
            self::MONTH_APRIL,
            self::MONTH_MAY,
            self::MONTH_JUNE,
            self::MONTH_JULY,
            self::MONTH_AUGUST,
            self::MONTH_SEPTEMBER,
            self::MONTH_OCTOBER,
            self::MONTH_NOVEMBER,
            self::MONTH_DECEMBER
        );

        return in_array($month, $list);
    }