Create subkey that will expire in 10 hours

[Kevin]

At 1365102936 seconds of The Epoch, Werner Koch wrote:
> Specifying the Epoch will anyway stop working in 2038 on many systems,
> thus it is probably not good to allow its use.  If a fixed data is
> required, one may always specify something like "20130404T153012" for
> both, the creation date and the expire date.

I *really* hope we have all the relevant systems patched by 2038, though
it's something of a testament to *nix stability that anyone seriously
considers the possibility that obsolete servers will still be up then.