Date validations are possible, but tricky. However, in your case there's a
catch. You can't validate the date until they (try) to submit the form...
Which means you can't do it at the moment they fill it in, only when they
click the Submit button.
Hmmm in that case, how do I make that happen?
Well, you need to create a function that submits the file with JS, and then add the date validation to that function.