How to generate an expired key with gpg?

Question

For testing purposes, I need a PGP key for a specific uid that has already expired.

Using gpg --full-generate-key only gives me the following options:

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

So the nearest expiration date I can set is tomorrow, which means that I'd have to wait 24 hours before I can perform my tests.

Is there a way to create an expired key other than changing the system date and time?

(I'm looking for a solution on macOS or Linux.)

Answer 1

You can set an expiration timestamp (in UTC) by typing something like “20220623T223000” for June 23rd, 2022, 22:30:00 UTC.

GnuPG will still consider that expiration timestamps in the past are invalid. It will however accept to generate a key that will expire in a few second.