public static String makeDate(int year, int month, int day) {
        if (year <= 1900 || year > 9999 || month <= 0 || day <= 0)
            return "Invalid arguments.";
        if( year % 4 == 0 && month == 2 && day <=29)
            return mkStr(year, month, day);
        if( month <= 12 && day <= monthDayMap[month-1])
            return mkStr(year, month, day);
        return "Invalid arguments.";
    }