code: private void zInternalSetLastValidDateAndNotifyListeners(LocalDate newDate) {
          LocalDate oldDate = lastValidDate;
          lastValidDate = newDate;
          if (!PickerUtilities.isSameLocalDate(oldDate, newDate)) {
              for (DateChangeListener dateChangeListener : dateChangeListeners) {
                  DateChangeEvent dateChangeEvent = new DateChangeEvent(this, oldDate, newDate);
                  dateChangeListener.dateChanged(dateChangeEvent);
              }
              // Fire a change event for beans binding.
              firePropertyChange("date", oldDate, newDate);
          }
      }
docs: zInternalSetLastValidDateAndNotifyListeners, This should be called whenever we need to change
  the last valid date variable. This will store the supplied last valid date. If needed, this
  will notify all date change listeners that the date has been changed. This does -not- update
  the displayed calendar, and does not perform any other tasks besides those described here.