Como @rob sugeriu nos comentários, libfaketime lida bem com isso:
$ gpg -v -v -v --batch --trust-model always -ear D5B9D38C <<< "qweqe"
gpg: using character set 'utf-8'
gpg: Note: signature key 077EA269D5B9D38C expired Sun 09 Nov 2014 12:57:25 PM +03
gpg: D5B9D38C: skipped: Unusable public key
gpg: [stdin]: encryption failed: Unusable public key
$ faketime '2012-12-24 08:15:42' gpg -v -v -v --batch --trust-model always -ear D5B9D38C <<< "qweqe"
gpg: using character set 'utf-8'
gpg: using subkey 85231B396B9569ED instead of primary key 077EA269D5B9D38C
gpg: No trust check due to '--trust-model always' option
gpg: reading from '[stdin]'
gpg: writing to stdout
gpg: RSA/AES256 encrypted for: "..."
-----BEGIN PGP MESSAGE-----
...
Resposta antiga:
- Iniciar UML (modo de usuário linux)
- Defina a data dentro da UML para um intervalo aceitável.
- Criptografar mensagem em UML