change memcache expire time

per expire in both php memcache or memcached:
http://www.php.net/manual/en/memcache.set.php
http://www.php.net/manual/en/memcached.expiration.php

This parameter means expiration time of the item. If it's equal to zero, the item will never expire. You can also use Unix timestamp or a number of seconds starting from current time, but in the latter case the number of seconds may not exceed 2592000 (30 days).

It's just how the memcached deal with this parameter. Please refer memcached.c from memcached source code. Function `realtime`

Currently it changes all the expire time to a absolute unix timestamp, and cannot set a relative time on memcached server. If the time on app server and memcached server differed a lot (one or both running on wrong time) will lead something unwanted.