Removing clutter from the internet
The internet in a simpler view is a huge storehouse of documents and information. However over time there has been a large buildup of files which are outdated or no longer useful. To help avoid this problem Google has implemented a new piece of code for WebPages, “Unavailable_after” meta tag which simply is a way to mark a page as no longer to be added to the net after a certain date. The advantages to such an inclusion would be web pages which could remove themselves from spider indexing when the information reaches a certain date making them self regulating.
However the truth is most webmasters will not use this when high demand keywords are in use. Most likely this will be used for smaller web pages and have little real impact.