There are 2 workarounds. One is just to update all components so it is visible that month has changed, which isn’t really user friendly. Another workaround is checking max day limit in DateModel#setModel and set it on max if current value is above maximum.

Here is sourcecode with includng changes (only AjaxFormComponentUpdatingBehavior, updateChoices, DateModel#setModel are changed):

public class DateChooser extends FormComponent {

/** The day ddc. */
private DropDownChoice dayDDC;

/** The month ddc. */
private DropDownChoice monthDDC;

/** The year ddc. */
private DropDownChoice yearDDC;

/**
* Instantiates a new date chooser.
*
* @param id
* the id
* @param model
* the model
*/
public DateChooser(final String id, IModel model) {
super(id);

if (model == null) {
model = new Model(new Date());
} else if (model.getObject() == null) {
model.setObject(new Date());
}

setModel(model);

monthDDC = new DropDownChoice(”month”, new DateModel(model, Calendar.MONTH), getMonths(), new IChoiceRenderer() {

public Object getDisplayValue(Integer object) {
SimpleDateFormat format = new SimpleDateFormat(”MMM”);

Calendar cal = Calendar.getInstance();
cal.set(Calendar.MONTH, object.intValue());

return format.format(cal.getTime());
}

public String getIdValue(Integer integer, int index) {
return String.valueOf(index);
}
});
monthDDC.add(new AjaxFormComponentUpdatingBehavior(”onchange”) {
@Override
protected void onUpdate(AjaxRequestTarget target) {
// change the days dropdown when month changes
updateChoices(target);
}
});
add(monthDDC);

yearDDC = new DropDownChoice(”year”, new DateModel(model, Calendar.YEAR), getYears());
add(yearDDC);
yearDDC.add(new AjaxFormComponentUpdatingBehavior(”onchange”) {
@Override
protected void onUpdate(AjaxRequestTarget target) {
// change the days dropdown when year changes
updateChoices(target);
}
});

dayDDC = new DropDownChoice(”day”, new DateModel(model, Calendar.DAY_OF_MONTH), getDays());
dayDDC.add(new AjaxFormComponentUpdatingBehavior(”onchange”) {

@Override
protected void onUpdate(AjaxRequestTarget target) {
}
});

dayDDC.setOutputMarkupId(true);
add(dayDDC);
}

protected void updateChoices(AjaxRequestTarget target) {
dayDDC.setChoices(getDays());
target.addComponent(dayDDC);
target.addComponent(yearDDC);
target.addComponent(monthDDC);
}

/**
* The Class DateModel.
*/
private class DateModel extends Model {

/** The date model. */
private final IModel dateModel;

/** The calendar field. */
private final int calendarField;

/**
* Instantiates a new date model.
*
* @param dateModel
* the date model
* @param calendarField
* the calendar field
*/
public DateModel(IModel dateModel, int calendarField) {
this.dateModel = dateModel;
this.calendarField = calendarField;
}

/*
* (non-Javadoc)
*
* @see org.apache.wicket.model.Model#detach()
*/
@Override
public void detach() {
dateModel.detach();
}

/*
* (non-Javadoc)
*
* @see org.apache.wicket.model.Model#getObject()
*/
@Override
public Integer getObject() {
if (dateModel.getObject() == null) {
return null;
}

Calendar cal = Calendar.getInstance();
cal.setTime(dateModel.getObject());

return cal.get(calendarField);
}

/*
* (non-Javadoc)
*
* @see org.apache.wicket.model.Model#setObject(java.io.Serializable)
*/
@Override
public void setObject(Integer object) {
Date date = dateModel.getObject();
if (date == null) {
date = new Date();
}
Calendar cal = Calendar.getInstance();
cal.setTime(date);

// if current day of the month is higher than allowed in new
// month/yer, set it to max allowed day in month
if (calendarField == Calendar.YEAR || calendarField == Calendar.MONTH) {
int totalDays = 31;
int year = yearDDC.getModelObject().intValue();
int month = monthDDC.getModelObject().intValue();
if (calendarField == Calendar.YEAR) {
year = object.intValue();
} else {
month = object.intValue();
}
Calendar cal2 = new GregorianCalendar(year, month, 1);
totalDays = cal2.getActualMaximum(Calendar.DAY_OF_MONTH);
if (dayDDC.getModelObject().intValue() > totalDays) {
dayDDC.setModelObject(Integer.valueOf(totalDays));
cal.set(Calendar.DAY_OF_MONTH, totalDays);
}
}

cal.set(calendarField, object);

dateModel.setObject(cal.getTime());
}
}

/**
* Gets the months.
*
* @return the months
*/
private List getMonths() {
List months = new ArrayList(12);

for (int i = 0; i < 12; i++) {
months.add(i);
}

return months;
}

/**
* Gets the days.
*
* @return the days
*/
protected List getDays() {
List days = new ArrayList(31);
int totalDays = 31;
if (yearDDC.getModelObject() != null && monthDDC.getModelObject() != null) {
Calendar cal = new GregorianCalendar(yearDDC.getModelObject(), monthDDC.getModelObject(), 1);
totalDays = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
}
for (int i = 1; i <= totalDays; i++) {
days.add(i);
}
return days;
}

/**
* Gets the years.
*
* @return the years
*/
protected List getYears() {
List years = new ArrayList(10);

Calendar cal = Calendar.getInstance();

for (int i = cal.get(Calendar.YEAR) – 2; i < cal.get(Calendar.YEAR) + 8; i++) {
years.add(i);
}

return years;
}

/*
* (non-Javadoc)
*
* @see org.apache.wicket.markup.html.form.FormComponent#convertInput()
*/
@Override
protected void convertInput() {
Calendar cal = Calendar.getInstance();
cal.set(yearDDC.getConvertedInput(), monthDDC.getConvertedInput(), dayDDC.getConvertedInput(), 0, 0, 0);
Date selectedDate = cal.getTime();
setModel(new Model(selectedDate));
}

}