How do I do this with OpenPGP?

From: John Dlugosz

One of the nice things about OpenPGP is that multiple signatories are
possible on a key, each "meaning" something.  Basically, it trent signs a
key, it's OK with me for (purpose A), and the fact that Carl signed it too
for some other purpose is beside the point.

But, I want Trent to be able to certify a key for a certain time period.
Tag 2, type 0x10-0x13 doesn't contain a date.  I suppose there's a more
complicated way to do this, though?  type 0x1F says "...for statements that
non-self certifiers want to make about the key itself" so maybe something
in there?  Or certifing one of the (time range) subkeys instead of the main
key?

Anyone?

--John