You can set key expiration to a date, by supplying a Unix timestamp instead of a number of days. This date can be more than 30 days in the future:
Expiration times are specified in unsigned integer seconds. They can be set from 0, meaning “never expire”, to 30 days (60*60*24*30). Any time higher than 30 days is interpreted as a unix timestamp date. If you want to expire an object on january 1st of next year, this is how you do that.
https://github.com/memcached/memcached/wiki/Programming#expiration
But, as you say, if you’re setting key expiration to an amount of time rather than a date, the maximum is 2,592,000 seconds, or 30 days.