private void fillBoxes() {
        for (int i = -20; i <= 20; i++) {
            cmbYear.addItem(calendar.get(Calendar.YEAR) + i);
        }
        for (int j = 1; j <= 12; j++) {
            cmbMonth.addItem(j);
        }
        cmbYear.setSelectedItem(year);
        cmbMonth.setSelectedItem(month);
        cmbYear.addItemListener(changeListener);
        cmbMonth.addItemListener(changeListener);
    }