It is considered good practice to set an expiration date on your GPG/PGP keys. Setting the expiration date prevents the possibility of losing the private key leaving a valid, non-expired, key out on the key servers that you cannot revoke. Revocation certificates may be one solution, but I found it more useful to simply set the expiration date.
It is useful if one can also remember how to extend the expiration date of the GPG key in question when the expiration date sneaks up without warning. Luckily, I was able to find an article by George Notaras on How to change the expiration date of a GPG key, which allowed me to change the date with minimal fuss.
The short version follows, but requires some reading up... so do so before execution.
$ gpg --edit-key [key-id-of-key-to-edit] gpg> key 0 gpg> expire ... gpg> key 1 gpg> expire