Reject year zero during datetime input, except when it's a 2-digit year

Reject year zero during datetime input, except when it's a 2-digit year
(then it means 2000 AD).  Formerly we silently interpreted this as 1 BC,
which at best is unwarranted familiarity with the implementation.
It's barely possible that some app somewhere expects the old behavior,
though, so we won't back-patch this into existing release branches.