Testing a metadata value before storing the content

 What we would like to do is allow a contributor to change the expiry date of a content item as long as the date they pick is one year from the dDateIn or less.  If they try to set the expiry date to longer than one year, the content server should refuse to accept the item.
I've been playing with rules, profiles and looking at the workflow but I'm not sure of the best method to test the field for valid dates before storing the item.

Anyone with some ideas, or do I write some javascript to roll into a component for the server?