http://yrral.net/cs230/web_development_basics/assignment2.html

Item 3: "One variable must be validated for something besides existence (for example, a number that has to be within a certain range)."

You could, for example, require that there be 31 days in January.

Also just a comment.  What you have is fine for the assignment, but for a real project, you would probably want to have boolean and a string to keep track of validity and error messages.  Then you could default your boolean to true, and any time a test fails, rather than alerting and returning false, you set valid = false and add an error message to your string.  At the end, if it is not valid, display your error.  Finally, return your boolean.  It doesn't have to be exactly like that, but it is good to list all the errors at once so the user doesn't have to keep reattempting to submit.