Howto: Changing the expiry date of GPG keys

"GnuPG keys can have an expiry date. When the key expires, it cannot be used to encrypt data anymore, and thus is a good way to enforce security measures. However, what most people does not seem to know is that this expiry date can be changed quite easily."

http://liquidat.wordpress.com/2013/05/07/howto-changing-the-expiry-date-of-gpg-keys/