Note that an expiration date on the master key doesn't really do anything for security, as anyone who compromised that could always just extend it anyway.  See http://madduck.net/blog/2006.06.20:expiring-gpg/ ([archived version](https://web.archive.org/web/20160629180203/http://madduck.net/blog/2006.06.20:expiring-gpg/)).

(Expiring subkeys, on the other hand, can be useful, and the option at key creation sets the date on all of the keys.)