Crystal is not a programming language. It will not validate your date string, it will just use it. You might create a formula that will read the parameter and if it incorrect will change the report title or show some text on the blank report, but there is no alert functionality.
A formula is the best that you can do, but if the data is a string, and the user entered the wrong value, the report will either a)display the valid data for the incorrect parameter or b)display nothing since nothing matches.