13.12 Cache Replacement

If a new cacheable (see Sections 14.9.2, 13.2.5, 13.2.6 and 13.8) response is received from a resource while any existing responses for the same resource are cached, the cache SHOULD use the new response to reply to the current request. It MAY insert it into cache storage and MAY, if it meets all other requirements, use it to respond to any future requests that would previously have caused the old response to be returned. If it inserts the new response into cache storage the rules in Section 13.5.3 apply.

Note: a new response that has an older Date header value than existing cached responses is not cacheable.