Installation fails if clock is in the past on ubuntu-keyring install

Installing from hoary Array 6 on a brand new laptop, the installation failed
when trying to install initrd-tools. VC3 showed that the actual problem was
ubuntu-base not installing because it depends on ubuntu-keyring, and
ubuntu-keyring didn't install because the keys in it were in the future. It
turned out that the hardware clock was in the past (set to Jan 1 2004; this was
a brand new laptop, and I didn't boot the Windows XP install on it first, I just
deleted it from the Ubuntu installation screens :)) and so that's why the keys
were in the future.

This entirely breaks installation from array 6 (and possibly subsequent
installs) in the above circumstance, hence the "major" severity.

Possible fixes:
Make ubuntu-keyring more tolerant of future-signed keys, but that might be a
problem in general
*If* ubuntu-keyring fails to install because keys are in the future, check with
the user that the clock is set right, and reinstall.

I don't know whether this is actually a bug in ubuntu-keyring-udeb -- is that
what the installer uses? If it is, implementing possible fix 1 above may be
easier, since it can be implemented in the udeb only?

It's possible that if the network was working, the correct time would have been
set from an NTP server, but the network wasn't working at this point.