symfony/symfony-docs — pull request #857 by lanthaler (2011-11-17 10:06:20+00:00)
Title: Corrected the wrong information that the Expires header is vulnerable to clock skew

This is not true as the lifetime calculation relies on the Date header. See http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-12#section-2.3.1