date based checkouts in git possible?

I'm in need of checking out the ports tree as it was
at an earlier date. But for the life of me, I can't
seem to cobble up an/the incantation.

All the suggestions I've read indicate something to
the effect of
git checkout `git rev-list -n 1 --before=<previous-date>" freebsd/main`
which returns
Your branch is up to date with 'freebsd/main'.

What must I do? Or will I need to convert/import it into svn?

Thanks!
Chris