Expires Meta Tag Description and Uses:
The expires meta tag indicates the date and time after which the content of the page is considered “expired”. It is intended to tell web spiders and robots when a page should be revisited to check for content changes. It also tells the browser how long it should keep the page in cache.
The contents of the expires meta tag must be in the RFC 2068 date format in Greenwich Mean Time (GMT) like this:
Day, dd Mon yyyy hh:mm:ss GMT
Sun, 28 Nov 2010 13:21:02 GMT
Expires Meta Tag Type:
The Expires meta tag is a http-equiv meta tag.
Expires Meta Tag Format:
<meta http-equiv="expires" content="Sun, 28 Nov 2010 13:21:02 GMT">
Expires Meta Tag Recommended Uses:
You can place an illegal date in the content to say that the page is expired immediately. This will also tell the browser to load the page from the server, not from a browser cache.
Most web spiders have their own rules for how often they revisit a page. So don’t expect that because you added the expires tag your page will be spidered more often.