you should pass the ModifiedDate through as part of the viewmodel, and then just overwrite it to DateTime.Today in the controller.
ie. prepopulate it the view
@Html.Hidden(Model.ModifiedDate,DateTime.Today);
and since this isn'tsecure, overwrite it in the controller
roleGroups.ModifiedDate = DateTime.Today;
or if its a universal field across your entire application (on all models), you can actually add custom code to do this in the mapper, but I'm unsure of the circumstances.