BUG: Fix broken YearBegin offset. Closes #2844.
This should close #2844. Good but unsettling news is it wasn't an implementation bug. The logic for handling months in the YearBegin offset just wasn't written. It was hard-coded to always be Year, 1, 1.

FYI, rebased.
 This breaks some unit tests.  Can you take a look?
 Yeah, I don't think the expectations for AS were correct in those tests.
 Thanks Skipper!
