Answer for Creative Exercise 2.1.30 at https://introcs.cs.princeton.edu/java/21function/

Calendar.java

public class Calendar {

    public static int getDayOfWeek(int day, int month, int year) {

        int monthTemp = (14 - month) / 12;
        
        int y0 = year - monthTemp;
        int x = y0 + (y0 / 4) - (y0 / 100) + (y0 / 400);
        
        int m0 = month + (12 * monthTemp) - 2;
        int d0 = (day + x + (31 * m0 / 12)) % 7;

        return d0;
    }

    public static String getMonthStr(int month) {

        String[] monthStr = {
            "January", "February", "March", "April", "May",
            "June", "July", "August", "September",
            "October", "November", "December"
        };

        return monthStr[month - 1];
    }

    public static boolean checkLeapYear(int year) {

        boolean isLeapYear;

        isLeapYear = year % 4 == 0;
        isLeapYear = isLeapYear && (year % 100 != 0);

        isLeapYear = isLeapYear || (year % 400 == 0);

        return isLeapYear;
    }

    public static int getDaysMonth(int month, int year) {

        int[] daysMonth = {31, 0, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

        if (checkLeapYear(year)) {

            daysMonth[1] = 29;

        } else {

            daysMonth[1] = 28;
        }

        return daysMonth[month - 1];
    }

    public static void printHeader(int month, int year) {

        System.out.println("     " + getMonthStr(month) + " " + year);
        System.out.println(" Su Mo Tu We Th Fr Sa");
    }

    public static void main(String[] argv) {

        int month = Integer.parseInt(argv[0]);
        int year = Integer.parseInt(argv[1]);

        int dayCount = 1;
        int dayInMonth = getDaysMonth(month, year);

        printHeader(month, year);
        
        for (int i = 0; i < 6; i++) {
            
            for (int j = 0; j < 7; j++) {
            
                if (getDayOfWeek(dayCount, month, year) == j && dayCount <= dayInMonth) {
                    
                    System.out.printf(" %2d", dayCount);
                    dayCount++;
                    
                } else {
                    
                    System.out.printf(" %2s", " ");
                }
            }
            
            System.out.println(" ");
        }
    }
}