The problem is that if it returns None on parse failure, then you can't tell the difference between the header not existing and the date not being parseable.  I don't have a solution for this problem.  Suggestions welcome.  (Note that this is only a problem in the new policy, where the parsing is done automatically; in the compat32 policy you have to apply parsedate yourself, so you can tell the difference between a non-existent header and a failed date parse).