public static function addDays($date, $days) {
        if (Validator::isDateTime($date) === false) throw new InvalidArgumentException('Invalid argument $date: '.$date);
        if (!is_int($days))                         throw new IllegalTypeException('Illegal type of parameter $days: '.gettype($days));

        $parts = explode('-', $date);
        $year  = (int) $parts[0];
        $month = (int) $parts[1];
        $day   = (int) $parts[2];

        return date('Y-m-d', mktime(0, 0, 0, $month, $day+$days, $year));
    }