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;
}