Comment #5 on issue 296 by peter.a....@gmail.com: Should SET operation returned STORED when unix time expiration has already past
http://code.google.com/p/memcached/issues/detail?id=296
So it appears OpenStack Swift uses absolute times because they allow users to set timeout values greater than 30 days, which is the limit used by memcached to determine if a timeout value is a delta or an absolute time. See the aforementioned bug if interested.
