code: function (picker, date) {
          var fields = this.fields,
              field = this.fields[picker.itemId];
  
          field.setChecked(true);
  
          if (field == fields.on) {
              fields.before.setChecked(false, true);
              fields.after.setChecked(false, true);
          } else {
              fields.on.setChecked(false, true);
              if (field == fields.after && this.getFieldValue('before') < date) {
                  fields.before.setChecked(false, true);
              } else if (field == fields.before && this.getFieldValue('after') > date) {
                  fields.after.setChecked(false, true);
              }
          }
          this.fireEvent('update', this);
  
          picker.up('menu').hide();
      }
docs: Handler for when the DatePicker for a field fires the 'select' event
  @param {Ext.picker.Date} picker
  @param {Object} date