java calendar return wrong month for the week

I want to get the last date for each of the X weeks that start from date Y.

for example the start date is 12/9/2011 and the test date is 15/9/2011 (which is in the first week starting from the start date).

Using the start and test dates, I would like to get the week number (week 1, week 2, week 3, etc...). For this case 15/9/2011 is in week 1 and the last date for week 1 is 18/9.

I wrote some java code to get the last dates for every week until week 19 starting from the start date, but it returns the wrong date.

How can i get the correct date?

import java.util.Calendar;

public class WeekCalculation {

 public static void main(String [] args) {
    Calendar dateStart = Calendar.getInstance();
    Calendar testDate1 = Calendar.getInstance();
    Calendar testDate2 = Calendar.getInstance();
    Calendar testDate3 = Calendar.getInstance();
    Calendar testDate4 = Calendar.getInstance();
    Calendar testDate5 = Calendar.getInstance();
    Calendar testDate6 = Calendar.getInstance();
    Calendar testDate7 = Calendar.getInstance();
    Calendar testDate8 = Calendar.getInstance();
    Calendar testDate9 = Calendar.getInstance();
    Calendar testDate10 = Calendar.getInstance();
    Calendar testDate11 = Calendar.getInstance();
    Calendar testDate12 = Calendar.getInstance();
    Calendar testDate13= Calendar.getInstance();
    Calendar testDate14 = Calendar.getInstance();
    Calendar testDate15 = Calendar.getInstance();
    Calendar testDate16 = Calendar.getInstance();
    Calendar testDate17 = Calendar.getInstance();
    Calendar testDate18 = Calendar.getInstance();
    Calendar testDate19 = Calendar.getInstance();
    dateStart.set(2011, 8, 12);

    testDate1.set(2011, 8, 15);
    testDate2.set(2011, 8, 20);
    testDate3.set(2011, 9, 1);
    testDate4.set(2011, 9, 7);
    testDate5.set(2011, 9, 13);
    testDate6.set(2011, 9, 20);
    testDate7.set(2011, 9, 29);
    testDate8.set(2011, 10, 4);
    testDate9.set(2011, 10, 12);
    testDate10.set(2011, 10, 17);
    testDate11.set(2011, 10, 25);
    testDate12.set(2011, 11, 2);
    testDate13.set(2011, 11, 8);
    testDate14.set(2011, 11, 17);
    testDate15.set(2011, 11, 20);
    testDate16.set(2011, 11, 27);
    testDate17.set(2012, 0, 7);
    testDate18.set(2012, 0, 13);
    testDate19.set(2012, 0, 20);

    int weekNumber1 = findWeekNumber(dateStart, testDate1);
    int weekNumber2 = findWeekNumber(dateStart, testDate2);
    int weekNumber3 = findWeekNumber(dateStart, testDate3);
    int weekNumber4 = findWeekNumber(dateStart, testDate4);
    int weekNumber5 = findWeekNumber(dateStart, testDate5);
    int weekNumber6 = findWeekNumber(dateStart, testDate6);
    int weekNumber7 = findWeekNumber(dateStart, testDate7);
    int weekNumber8 = findWeekNumber(dateStart, testDate8);
    int weekNumber9 = findWeekNumber(dateStart, testDate9);
    int weekNumber10 = findWeekNumber(dateStart, testDate10);
    int weekNumber11 = findWeekNumber(dateStart, testDate11);
    int weekNumber12 = findWeekNumber(dateStart, testDate12);
    int weekNumber13 = findWeekNumber(dateStart, testDate13);
    int weekNumber14 = findWeekNumber(dateStart, testDate14);
    int weekNumber15 = findWeekNumber(dateStart, testDate15);
    int weekNumber16 = findWeekNumber(dateStart, testDate16);
    int weekNumber17 = findWeekNumber(dateStart, testDate17);
    int weekNumber18 = findWeekNumber(dateStart, testDate18);
    int weekNumber19 = findWeekNumber(dateStart, testDate19);

    findDayLeftInTheWeek(dateStart, weekNumber1, testDate1);
    findDayLeftInTheWeek(dateStart, weekNumber2, testDate2);
    findDayLeftInTheWeek(dateStart, weekNumber3, testDate3);
    findDayLeftInTheWeek(dateStart, weekNumber4, testDate4);
    findDayLeftInTheWeek(dateStart, (weekNumber5), testDate5);
    findDayLeftInTheWeek(dateStart, weekNumber6, testDate6);
    findDayLeftInTheWeek(dateStart, weekNumber7, testDate7);
    findDayLeftInTheWeek(dateStart, weekNumber8, testDate8);
    findDayLeftInTheWeek(dateStart, weekNumber9, testDate9);
    findDayLeftInTheWeek(dateStart, weekNumber10, testDate10);
    findDayLeftInTheWeek(dateStart, (weekNumber11), testDate11);
    findDayLeftInTheWeek(dateStart, weekNumber12, testDate12);
    findDayLeftInTheWeek(dateStart, weekNumber13, testDate13);
    findDayLeftInTheWeek(dateStart, weekNumber14, testDate14);
    findDayLeftInTheWeek(dateStart, weekNumber15, testDate15);
    findDayLeftInTheWeek(dateStart, weekNumber16, testDate16);
    findDayLeftInTheWeek(dateStart, (weekNumber17), testDate17);
    findDayLeftInTheWeek(dateStart, weekNumber18, testDate18);
    findDayLeftInTheWeek(dateStart, weekNumber19, testDate19);
}

public static int findWeekNumber(Calendar startDate, Calendar currentDate) {
    long milliseconds1 = startDate.getTimeInMillis();
    long milliseconds2 = currentDate.getTimeInMillis();
    long diff = milliseconds2 - milliseconds1;;
    long diffDays = diff / (24 * 60 * 60 * 1000);
    return (int)(diffDays/7)+1;
}

public static void findDayLeftInTheWeek(Calendar startDate, int weekNumber, Calendar currentDate) {
    Calendar lastDateOfTheWeek = Calendar.getInstance();
    long milliseconds1 = startDate.getTimeInMillis();
    long oneday = 24*60*60*1000;
    long lastDate = (weekNumber * 7 * 24 * 60 * 60 * 1000)+milliseconds1;
    lastDateOfTheWeek.setTimeInMillis(lastDate-oneday);
    System.out.println("last date of the week "+weekNumber+ " " +lastDateOfTheWeek.get(Calendar.DATE)+"/"+(lastDateOfTheWeek.get(Calendar.MONTH)+1)+"/"+lastDateOfTheWeek.get(Calendar.YEAR));
}
}

Integer overflow

The problem is you use integer variables to calculate some numbers that can exceed the Integer.MAX_VALUE

Check the part of this code inside the parenthesis, you will notice it actual is -1875767296 in week 4, that's why your date decrease as you increase the week number

long lastDate = (weekNumber * 7 * 24 * 60 * 60 * 1000) + milliseconds1;

Since Java 8, you can use the …Exact methods in Math class to trap for an ArithmeticException on integer overflow.