Google is coming out with a new tag called “unavailable_after” which will allow people to tell Google when a particular page will no longer be available for crawling.
For instance, if you have a special offer on your site that expires on a particular date, you might want to use the unavailable_after tag to let Google know when to stop indexing it. Or perhaps you write articles that are free for a particular amount of time, but then get moved to a paid-subscription area of your site. Unavailable_after is the tag for you! Pretty neat stuff!