I am attaching datetime-sandbox-pypy.diff, a plain diff between six-year-old sandbox and pypy versions. (Plain diff is cleaner than unified diff.)
You can see that the differences are trivial. I notice, however that original datetime implementation was returning subclass instances from operations on datetime subclass instances. Brett, this is off-topic hear, but I would appreciate your take on msg107410.
BTW, in order to preserve history, it may be a good idea to develop this in a branch off datetime sandbox and merge it back when ready.