The Expires entity-header field gives a date and time after which the description or media-stream should be considered stale. The interpretation depends on the method:
Describe response: The Expires header indicates a date and time after which the description should be considered stale.
A stale cache entry may not normally be returned by a cache (either a proxy cache or an user agent cache) unless it is first validated with the origin server (or with an intermediate cache that has a fresh copy of the entity). See section 13 for further discussion of the expiration model.
The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time.
To mark a response as "already expired," an origin server should use an Expires date that is equal to the Date header value. To mark a response as "never expires," an origin server should use an Expires date approximately one year from the time the response is sent. RTSP/1.0 servers should not send Expires dates more than one year in the future.
The presence of an Expires header field with a date value of some time in the future on a media stream that otherwise would by default be non-cacheable indicates that the media stream is cacheable, unless indicated otherwise by a Cache-Control header field.

public class Expires : BaseHeader

Public Class Expires
	Inherits BaseHeader

public ref class Expires : public BaseHeader

type Expires =  
    class
        inherit BaseHeader
    end