bug #2400: Fix expiration time for mencache.

When the expiration time specified is 30 days or more in seconds, Memcache treats the expiration as an absolute date by converting the amount of seconds specified to a Unix epoch date. Be careful, because specifying 40 days in seconds will set the expiration to a time in 1970, which will yield unknown results.(cherry picked from commit 20831c763400221567d3119131c2311497c71808)