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Thanks for the comments...

1)  Please put *all* the code for the C API into
datetimeapi.h. I don't like
your current mix of putting some things into datetime.h and
others into
datetimeapi.h (e.g. the PyDateTime_CAPI definition).

2) Since it's the only API missing if you want to use
datetime objects
in database interfacing, please add it.

3) dito.

4) Yes, the header file is the right place. Have a look at
unicodeobject.h
for what I have in mind (or mxDateTime.h for that matter).

5) Why add yet another magic number ? Isn't the Python API
number
enough ?

6) Since you don't provide a doc patch, please put comments into
the header file. There can never be enough documentation and 
developers tend to look at the code rather than the docs ;-)