You are best of to create a microflow that does a check on dateAttribute > [%CurrentDateTime%] if this is false then show validation feedback.
You can trigger this microflow either on the onChange of the date selector to give direct feedback or on a custom save button. You could also trigger this microflow on a before commit event of the object, but in this case i don't think i would obt for that as this is not something that should always be true.