code: @Override
      public void onDateChanged(int year, int month, int day) {
          mCalendar.set(year, month, day);
  
          updateDateTab();
      }
docs: <p>The callback used by the DatePicker to update {@code mCalendar} as
  the user changes the date. Each time this is called, we also update
  the text on the date tab to reflect the date the user has currenly
  selected.</p>
  <p>
  <p>Implements the {@link DateFragment.DateChangedListener}
  interface.</p>