Today we had a problem at work on a system. Without getting into too much detail as to give away secrets behind the verbal NDA I am behind, I will just say that it had to do with a GPG public key of mine that was expired on a dev machine, accidentally propagating during install to a production machine. This key had a sub key as well, so figuring out this was tricky.

To start, you can list your gpg keys like so:

$ gpg --list-keys

This will list keys such as

pub 4096R/01A53981 2011-11-09 [expires: 2016-11-07]
uid Tyrel Anthony Souza (Five year key for email.)
sub 4096R/C482F56D 2011-11-09 [expires: 2016-11-07]

To make this not expire, (same steps to change expiration date to another time), you must first edit the key

$ gpg --edit-key 01A53981

You will then see a gpg prompt gpg>

Type "expire" in and you will be prompted for how long to change it to

Changing expiration time for the primary key.
Please specify how long the key should be valid.
0 = key does not expire
<n> = key expires in n days
<n>w = key expires in n weeks
<n>m = key expires in n months
<n>y = key expires in n years

You are then done setting the expiration on the primary key, if you have sub key, doing this is as easy as typing key 1 and repeating the expiration step.

To finish and wrap things up, type save and you are done.