The HTTP If-Modified-Since Header, is used by user agents when sending a GET request, to check whether a requested resource has changed since the last visit. If the resource hasn't been changed since the last visit, a 304 not modified response is returned, and the cached version is used instead.
This header can be used alone, or in combination with other caching headers, to increase site performance on repeat views, and to lower bandwidth usage.
The date format of the If-Modified-Since Header is as follows:
DD-Mon-YY HH:MM:SS GMT
Note. GMT must be used as the timezone.
An example of the If-Modified-Since field:
If-Modified-Since: Mon, 29 Nov 2010 09:47:40 GMT