The WEEK_OF_YEAR numbering in GregorianCalendar assumes the week-based year numbering. Therefore, the combination of year (Gregorian), week of year and day of week doesn't specify a unique date. Also, setting WEEK_OF_YEAR value to the same one may change the date.

The following is a program to reproduce the problem.

--
import java.util.*;

public class WeekNumbering {

    static GregorianCalendar gcal = new GregorianCalendar();

    public static void main(String[] args) {
        printDate(2004, Calendar.JANUARY, 1);
        printDate(2004, Calendar.DECEMBER, 30);

        gcal.set(gcal.WEEK_OF_YEAR, gcal.get(gcal.WEEK_OF_YEAR));
        System.out.println(gcal.getTime() + ": week# " +
                        gcal.get(gcal.WEEK_OF_YEAR));
    }

    static void printDate(int year, int month, int dom) {
        gcal.clear();
        gcal.set(year, month, dom);
        System.out.println(gcal.getTime() + ": week# " +
                        gcal.get(gcal.WEEK_OF_YEAR));
    }
}
--
The output in en_US locale:

Thu Jan 01 00:00:00 PST 2004: week# 1
Thu Dec 30 00:00:00 PST 2004: week# 1
Thu Jan 01 00:00:00 PST 2004: week# 1


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