P3: Somewhat important
This is expiration calculation in RFC 13.2.4.
"If none of Expires, Cache-Control: max-age, or Cache-Control: s- maxage (see section 14.9.3) appears in the response, and the response does not include other restrictions on caching, the cache MAY compute a freshness lifetime using a heuristic. The cache MUST attach Warning 113 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%."
Heuristic with Last-Modified time in Qt does not work well.
This is heuristic code in QNetworkReplyHttpImplPrivate::loadFromCacheIfAllowed function.
Diff is negative value, because last-modified is earlier than current time.
Because diff is negative value, Expiration date will be earlier than last-modified.
ExpirationDate is earlier than dateHeader, and freshness_lifetime is also negative value.
So response_is_fresh value is false. It means that disk cache heuristic does not cache files.