The "unavailable_after" tag will allow you to tell Google when Googlebot should no longer crawl that page. Jill explains that this tag comes in handy when you have a limited time offer promotional page, and on this page, the promotion will expire on a specific date. By using the "unavailable_after" tag, you can tell Google that they should not crawl this page, after the promotion expires. There are several practical scenarios for this tag. We have emailed Dan Crow and others at Google for when this tag is expected to be supported.