Apps Change Date from Variables

[Duny]

I am trying to get the Day_of_year from a Datepicker, works but I want to alsom change the day of the year, is there a function like in PHP to change the values.

    final Calendar c = Calendar.getInstance();
    mYear = c.get(Calendar.YEAR);
    mMonth = c.get(Calendar.MONTH);
    mDay = c.get(Calendar.DAY_OF_MONTH);
    jDay = c.get(Calendar.DAY_OF_YEAR);

    // display the current date (this method is below)
    updateDisplay();
}
private void updateDisplay() {
    mDateDisplay.setText(
        new StringBuilder()
                // Month is 0 based so add 1
                .append(jDay));
private DatePickerDialog.OnDateSetListener mDateSetListener = new DatePickerDialog.OnDateSetListener() {

            public void onDateSet(DatePicker view, int year, 
                                  int monthOfYear, int dayOfMonth) {
                mYear = year;
                mMonth = monthOfYear;
                mDay = dayOfMonth;
                jDay = 123;//need this to change, to get the values from mYear, mMonth and mDay to change the day of the year (jDay).
                updateDisplay();
            }
        };