What's wrong with my code? Zeller's Congruence algorithm code

Question

What's wrong with my code? For some reason I keep on getting the day off by one day? For example, today is the 26th of 2013 and it's a Tuesday, but the program tells me it's a Wednesday. I am using Zeller's Congruence algorithm.

import javax.swing.JOptionPane;
public class zeller {
    public static void main(String[] args) {
        String yearString = JOptionPane.showInputDialog("Enter the year:");

        int year = Integer.parseInt(yearString);

        String monthString = JOptionPane.showInputDialog("Enter the month (3-12)(January and Feburary are 13 and 14):");
        int month = Integer.parseInt(monthString);

        String dayString = JOptionPane.showInputDialog("Enter the day 1-31: ");  
        int day = Integer.parseInt(dayString);

        switch (month) {
            case 13:  monthString = "January";
                break;
            case 14:  monthString = "February";
                break;
            case 3:  monthString = "March";
                break;
            case 4:  monthString = "April";
                break;
            case 5:  monthString = "May";
                break;
            case 6:  monthString = "June";
                break;
            case 7:  monthString = "July";
                break;
            case 8:  monthString = "August";
                break;
            case 9:  monthString = "September";
                break;
            case 10: monthString = "October";
                break;
            case 11: monthString = "November";
                break;
            case 12: monthString = "December";
                break;
            default: monthString = "Invalid month";
                break;
        }

        int j = year / 100;
        int k = year % 100 ;
        double h = (day + ((26*(month + 1)) / 10) + k + (k / 4) +(j / 4) + (5 * j)) % 7;
        int h1 = (int)h;

        switch (h1) {
            case 0:  dayString = "Saturday";
                break;
            case 1:  dayString = "Sunday";
                break;
            case 2:  dayString = "Monday";
                break;
            case 3:  dayString = "Tuesday";
                break;
            case 4:  dayString = "Wednesday";
                break;
            case 5:  dayString = "Thursday";
                break;
            case 6:  dayString = "Friday";
                break;

            default: monthString = "Invalid month";
                break;
        }

        System.out.println("Day of the week is: " + dayString);
    }
}

Answer 1

1. The months Jan and Feb are counted as the 13th and 14th month of the previous year, so if you want the user to enter the actual months of 1 or 2 for Jan or Feb you could add code like this:

if (month == 1 ){
    month = 13;
     year -=1;
 }
else if (month == 2) {
     month = 14;
     year -=1;
}

Answer 2

From Wikipedia,

In this algorithm January and February are counted as months 13 and 14 of the previous year. E.g. if it is February 2, 2010, the algorithm counts the date as the second day of the fourteenth month of 2009 (02/14/2009 in DD/MM/YYYY format).

So there is technically nothing wrong with your code; putting in 02/26/2013 (today) as the 26th day of the 14th month of 2013 is actually calculating the day of the week of 02/26/2014.

Answer 3

You need to modify the year if it is January or February. Explanation on wikipedia:

One can readily see that, in a given year, March 1 (if that is a Saturday, then March 2) is a good test date; and that, in a given century, the best test year is that which is a multiple of 100. Zeller used decimal arithmetic, and found it convenient to use J and K in representing the year. But when using a computer, it is simpler to handle the modified year Y, which is Y - 1 during January and February:

I modified your code as follows and it works:

import javax.swing.JOptionPane;

public class zeller {
    public static void main(final String[] args) {

        String yearString = JOptionPane.showInputDialog("Enter the year:");

        int year = Integer.parseInt(yearString);

        String monthString =
            JOptionPane.showInputDialog("Enter the month (3-12)(January and Feburary are 13 and 14):");
        int month = Integer.parseInt(monthString);

        String dayString = JOptionPane.showInputDialog("Enter the day 1-31: ");
        int day = Integer.parseInt(dayString);

        switch (month) {
        case 14:
            year--;
            monthString = "January";
            break;
        case 13:
            year--;
            monthString = "February";
            break;
        case 3:
            monthString = "March";
            break;
        case 4:
            monthString = "April";
            break;
        case 5:
            monthString = "May";
            break;
        case 6:
            monthString = "June";
            break;
        case 7:
            monthString = "July";
            break;
        case 8:
            monthString = "August";
            break;
        case 9:
            monthString = "September";
            break;
        case 10:
            monthString = "October";
            break;
        case 11:
            monthString = "November";
            break;
        case 12:
            monthString = "December";
            break;
        default:
            monthString = "Invalid month";
            break;
        }
        int j = year / 100;
        int k = year % 100;
        double h = (day + ((13 * (month + 1) / 5)) + k + (k / 4) + (j / 4) + (5 * j)) % 7;
        int h1 = (int) h;
        switch (h1) {
        case 0:
            dayString = "Saturday";
            break;
        case 1:
            dayString = "Sunday";
            break;
        case 2:
            dayString = "Monday";
            break;
        case 3:
            dayString = "Tuesday";
            break;
        case 4:
            dayString = "Wednesday";
            break;
        case 5:
            dayString = "Thursday";
            break;
        case 6:
            dayString = "Friday";
            break;

        default:
            monthString = "Invalid month";
            break;
        }

        System.out.println("Day of the week is: " + dayString);

    }
}