unavailable_after tag - Google Robots Exclusion Protocol

The ‘unavailable_after’ meta tag will soon be recognized by Google according to Dan Crow, Director of Crawl Systems at Google. from Loren Baker

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.

Two new features added to the protocol will help webmasters govern
when an item should stop showing up in Google’s web search, as well
as providing some control over the indexing of other data
types.

One of the features, support for the unavailable_after tag, has
been mentioned previously. Google’s Dan Crow made that initial
disclosure.

He has followed that up with a full-fledged post on the official
Google blog about the new tag. The unavailable_after META tag
informs the Googlebot when a page should be removed from Google’s
search results:

“This information is treated as a removal request: it will take
about a day after the removal date passes for the page to disappear
from the search results. We currently only support unavailable_after
for Google web search results.”

“After the removal, the page stops showing in Google search results
but it is not removed from our system.”
(Email from: David A. Utter)

One of the major issues plaguing search engines right now is the growing list of web documents available online. While no exact numbers are available, there are billions of search results to sort through. But, they can’t all be relevant both on material content and time — can they?

Of course they’re not, and Google is hoping to solve this problem through the adoption of the unavailable_after META tag. more here
(From Sujan Patel: SEO Impact of Google’s unavailable_after META Tag)

Source :http://www.searchengineoptimizationcompany.ca