[pca] specifying a specific date instead of minage

[amy.rich at tufts.edu]

bliss.kevin> You can either make a local copy of the patchdiag.xref on all of
bliss.kevin> the servers on the same day.

But then you're once again limited to running pca with --nocheckxref or it
will update whenever you run, say, pca -l.  The time between patching on dev
and production can be as much as three months or sometimes more.

bliss.kevin> Or can run pca -d (whatever other flags you use) to download
bliss.kevin> instead of install on the same day on all of the servers.

That defeats the purpose of having a central patch repository and takes up too
much space on the thousands of client nodes.  It also doesn't delete the
patches once it's installed them, so you'd have to manually clean them out
after you've finished a patch cycle.