GWT datepicker group for seasons calendar

// Date picker overriden
public class RangoDatePicker extends DatePicker {

    private final static DateTimeFormat monthFormat = DateTimeFormat.getFormat(PredefinedFormat.YEAR_MONTH);

    public RangoDatePicker(int month, int year, RangoDatePickerGroup rangoDatePickerGroup) {
    super(new MonthSelector() {
        private Grid grid;

        @Override
        protected void refresh() {
        String formattedMonth = getModel().formatCurrentMonth();
        grid.setText(0, 0, formattedMonth);
        }

        @Override
        protected void setup() {
        // Set up grid.
        grid = new Grid(1, 1);

        CellFormatter formatter = grid.getCellFormatter();
        formatter.setStyleName(0, 0, "datePickerMonth");
        formatter.setWidth(0, 0, "100%");
        grid.setStyleName("datePickerMonthSelector");
        initWidget(grid);
        }
    }, new DefaultCalendarView(), new CalendarModel() {
        @Override
        public String formatCurrentMonth() {
        return monthFormat.format(getCurrentMonth());
        }
    });

    // style
    addStyleName("rango-cal");

    // group
    rangoDatePickerGroup.addRangoCalendar(month, year, this);

    // set current month
    Date currentMonth = new GregorianCalendar(year, month, 1).getTime();
    this.setCurrentMonth(currentMonth);
    }

}

//Date picker group
public class RangoDatePickerGroup {
    static HashMap<String, RangoDatePicker> datePickers = new HashMap<String, RangoDatePicker>();

    public void addRangoCalendar(int month, int year, RangoDatePicker rangoDatePicker) {
    datePickers.put(key(year, month), rangoDatePicker);
    }

    private String key(int year, int month) {
    return year + "-" + month;
    }

    public void selectDates(Date start, Date end, int colorCode) {
    for (final Date date = start; date.compareTo(end) <= 0; CalendarUtil.addDaysToDate(date, 1)) {

        @SuppressWarnings("deprecation")
        /* Yes, it's deprecated */
        final int month = date.getMonth();
        final int year = date.getYear();

        datePickers.get(key(year, month)).addStyleToDates("rango-colour"+colorCode, date);
    }
    }

}

//CSS

.rango-cal .datePickerDay.datePickerDayIsFiller{
    visibility: hidden;
}
.rango-cal .datePickerDay.datePickerDayIsWeekend{
    background: transparent;
}
.rango-cal .datePickerDay.datePickerDayIsValue{
    background: transparent;
}
.rango-cal .datePickerDay.rango-colour-0 {
    background-color: green;
}