Abort Form Save if Date Field outside of range?

This answer is slightly different if you're on new forms or legacy - but in the 'Actions' part of the form rule there are two things:

1. Prevent Save - so this wouldn't pop up with the message until you hit save but the action for prevent save will have a text input for your commentary
2. Display Message and you can pick the type (modal, notification, field) that will fire immediately when your condition is met. So if you trigger the rule on the checkbox changing to true meaning their entry is now wrong it'll pop up the message.