//raise the property changed for CurrentlySelectedDateProperty
static void CurrentlySelectedDatePropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
DatePicker picker = (DatePicker)obj;
picker.OnDateChanged(picker.CurrentlySelectedDate, (DateTime)e.OldValue);
picker.OnPropertyChanged(new PropertyChangedEventArgs(CurrentlySelectedDateStringPropertyName));
}