What do you mean by "validation model", exactly? You can set a up a date format for a field, either using one of the built-in ones or create a Keystroke/Format JavaScript yourself, or you could also use a custom Validation JavaScript. Another option is to use a script to check the fields for correctness when the submit button is clicked. With a custom script, you can provide the user more information in the error message. If you're using a custom script, it would be helpful to see what it is and we can suggest some revisions.