Web pages do not usually have date information. This can lead people into believing that what they have read is current (see previous post).
Most printed material have publishing dates so why not websites.
Of course there is the Wayback Machine: www.archive.org which allows you to browse through 30 billion web pages archived from 1996 to a few months ago.
Another trick I’ve just discovered it to type:
into the address bar of a page – it works sometimes but probably not for database driven sites (ie blogs, myspace etc).