Check Date....

I have a form input object where users enter dates. I currently have a JavaScript that autoformats and checks the dates. The code for this JavaScript can be found at the end of this posting. I was e-mailed the code by a friend and have been using it for quiet sometime. It is useful.
Anyhow, I would like to add a check that will prevent entering a date prior to today and after a specific date.
Any help would be appreciated.

is the date fixed? a certain number of days away? where are you getting the original date from? (it needs to be server side originally in order to prevet someone changing their system clock and manipulating your code)