Expires - provides information for managing caching. If the date in the header has Expiresnot yet occurred, the browser can display the cached copy of the page. If the specified date is in the past, then the browser should, although not obliged, download the page from the server. If you initially specify the past date, then the document may not be cached, that is, in theory it should provoke the browser to contact the server for a more recent version.
Some search engines can refuse to index the document with an obsolete date.
The date must be specified in the standard [RFC850].
< meta http-equiv = "Expires" content = "Wed, 26 Feb 1999 09:00:00 GMT" >