code: @Override
      protected void buildContent() {
  
          for (int i = 0, n = this.getCalendarSystem().getMaxCountOfMonths(); i < n; i++) {
              Button button = new Button();
              button.getStyleClass().add(CSS_CALENDAR_CELL_INSIDE_RANGE);
  
              button.setMaxWidth(Double.MAX_VALUE);
              button.setMaxHeight(Double.MAX_VALUE);
              GridPane.setVgrow(button, Priority.ALWAYS);
              GridPane.setHgrow(button, Priority.ALWAYS);
  
              button.setOnAction(
                  actionEvent -> {
                      if (getControl().viewIndexProperty().get() == NavigationBar.YEAR_VIEW) {
                          T clickedDate = getControl().chronology().getChronoType().cast(button.getUserData());
                          getControl().pageDateProperty().setValue(clickedDate);
                          getControl().viewIndexProperty().set(NavigationBar.MONTH_VIEW);
                      }
                  }
              );
  
              int colIndex = i % NUM_OF_COLUMNS;
              int rowIndex = i / NUM_OF_COLUMNS;
              add(button, colIndex, rowIndex);
          }
  
      }
docs: ~ Methoden ----------------------------------------------------------