public static function renderAge(DateTime $birthDate, DateTime $refDate = null) {

        // Use the current date/time.
        if (null === $refDate) {
            $refDate = new DateTime();
        }

        $diff  = $refDate->getTimestamp() - $birthDate->getTimestamp();
        $years = new DateTime("@" . $diff);

        return intval($years->format("Y")) - 1970;
    }