Why do you need a pure Python version of datetime:
 - it's easier to work on a new feature first in Python: there are more people are able to write Python code, than people able to write C code (especially C code using the Python C API)
 - it helps datetime debuging
 - it helps other Python implementations (not only PyPy)
 - it improves the quality of the tests and so of the C version

I think that the first point is the most important, but datetime still lack many feature and is far from being perfect.

I don't think that the pure Python implementation should be used by default: the current C implementation should stay because it's faster and many people use it. I don't know the best name for the Python version, maybe pydatetime.py (or _pydatetime.py).

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Questions:

 - @Alexander: did you contacted people from IronPython and Jython?
 - Is the Python version compatible with the C version about the serialization (pickle)?

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r.david.murray> there may be more important stuff to python

You cannot force other developer to work on a specific topic since Python is only developed by hackers in their free time. If Alexander would like to work on this, he have to do it :-)