Expires is a TDateTime property that represents the date and time after which the content of an entity is considered stale. A stale cache entry may not normally be returned by a cache unless it is first validated with the origin server. The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after the indicated timestamp value. 

Expires is read from the headers in an HTTP response, and may originally contain either an integer number of seconds for the expiration value or an actual date and time. When the header is expressed as an integer value, the Expires property will be given a value that corresponds to the current date and time plus the number of seconds in the header. 

A response that includes a CacheControl header with the max-age directive will override the Expires property.

CacheControl