code: void onDateGroupChanged(@Observes DataSetGroupDateChanged event) {
          ColumnGroup columnGroup = event.getColumnGroup();
          DataSetGroup groupOp = getFirstGroupOp();
          if (groupOp != null) {
              groupOp.setColumnGroup(columnGroup);
  
              changeEvent.fire(new DataSetLookupChangedEvent(dataSetLookup));
          }
      }
docs: DataSetGroupDateEditor events