I would simply email their developer mailing list (find it at http://pypy.org/contact.html) and say that you are willing to work on this. Maciej and I have discussed this before, so this won't be a total shock to them.
As for Raymond's comment, I think he understood what I meant. What he is worried about is that datetime as PyPy has implemented it is done in RPython which is a custom subset of Python, and not a "normal" Python implementation. But if they have simply been maintaining the pure Python version that Tim wrote way back in the day then I suspect it's not in RPython.