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 [...]
From: http://www.mattcutts.com/blog/whats-an-update/ (Normally I, you know, think before I post. I’m experimenting with the quick-post-with-very-little-thought technique here.) What is an update? Google updates its index data, including backlinks and PageRank, continually and continuously. We only export new backlinks, PageRank, or directory data every three months or so though. (We started doing that last year when [...]