Implement a notification for back dated assignment changes

we need to implement a notification for the back-dated assignment changes. When a user do a assignment change for a back date, there should be a pop-up message as "Future Dated Assignment Has Been Placed ".

E.X - System has an assignment change on 5th of January and we try to update another assignment for 01st of January. In that point there should be a system notification “Future Dated Assignment Has Been Placed” or a formal message.