The date and time after which the document should be considered expired. Controls cacheing in HTTP/1.0. In Netscape Navigator, a request for a document whose expires time has passed will generate a new network request (possibly with If-Modified-Since). An illegal Expires date, e.g. "0", is interpreted as "now". Setting Expires to 0 may thus be used to force a modification check at each visit.
Web robots may delete expired documents from a search engine, or schedule a revisit.
Dates must be given in RFC850 format, in GMT. E.g. (META tag):
<META HTTP-EQUIV="expires" CONTENT="Wed, 26 Feb 1997 08:21:57 GMT">
or (HTTP header):
Expires: Wed, 26 Feb 1997 08:21:57 GMT
In HTTP 1.0, an invalid value (such as "0") may be used to mean "immediately".
Note: While the Expires HTML META tag appears to work properly with Netscape Navigator, other browsers may ignore it, and it is ignored by Web proxies. Use of the equivalent HTTP header, as supported by e.g. Apache, is more reliable.
