The Last-Modified entity-header field indicates the date and time at which the sender believes the resource was last modified. The exact semantics of this field are defined in terms of how the recipient should interpret it: if the recipient has a copy of this resource which is older than the date given by the Last-Modified field, that copy should be considered stale.
Last-Modified = "Last-Modified" ":" HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The exact meaning of this header field depends on the implementation of the sender and the nature of the original resource. For files, it may be just the file system last-modified time. For entities with dynamically included parts, it may be the most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update timestamp of the record. For virtual objects, it may be the last time the internal state changed.
An origin server must not send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's last modification would indicate some time in the future, the server must replace that date with the message origination date.