QNetworkDiskCache: expiration calculation with Last-Modified time not working well

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.

// RFC 2616 13.2.4 Expiration Calculations
if (!expirationDate.isValid()) {
    if (lastModified.isValid()) {
        int diff = currentDateTime.secsTo(lastModified);
        expirationDate = lastModified.addSecs(diff / 10);
        if (httpRequest.headerField("Warning").isEmpty()) {
             QDateTime dt;
             dt.setTime_t(current_age);
             if (dt.daysTo(currentDateTime) > 1)
                 httpRequest.setHeaderField("Warning", "113");
        }
    }
}
// the cache-saving code below sets the expirationDate with date+max_age
// if "max-age" is present, or to Expires otherwise
int freshness_lifetime = dateHeader.secsTo(expirationDate);
response_is_fresh = (freshness_lifetime > current_age);

Diff is negative value, because last-modified is earlier than current time.
(*. http://qt-project.org/doc/qt-5/qdatetime.html#secsTo)
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.