In order to decide whether a response is fresh or stale, we need to compare its freshness lifetime to its age. The age is calculated as described in section 13.2.3; this section describes how to calculate the freshness lifetime, and to determine if a response has expired. In the discussion below, the values can be represented in any form appropriate for arithmetic operations.
We use the term "expires_value" to denote the value of the Expires header. We use the term "max_age_value" to denote an appropriate value of the number of seconds carried by the max-age directive of the Cache-Control header in a response (see section 14.10.
The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:
freshness_lifetime = max_age_value
Otherwise, if Expires is present in the response, the calculation is:
freshness_lifetime = expires_value - date_value
Note that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server.
If neither Expires nor Cache-Control: max-age appears in the response, and the response does not include other restrictions on caching, the cache MAY compute a freshness lifetime using a heuristic. If the value is greater than 24 hours, the cache must attach Warning 13 to any response whose age is more than 24 hours if such warning has not already been added.
Also, if the response does have a Last-Modified time, the heuristic expiration value SHOULD be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
The calculation to determine if a response has expired is quite simple:
response_is_fresh = (freshness_lifetime > current_age)