Did you see my class attributes suggestion a couple messages back? That might solve your dispatch problem:

def _parse_isodatetime(cls, string):
    match = cls._iso_regex.match(...)

class time:
    _iso_regex = re.compile(...)

# or

time._iso_regex = re.compile(...)
date._iso_regex = ...

For the C module, start looking at /Modules/datetimemodule.c. Maybe if you can think of a similar function implemented in C to copy off. It looks like strptime() defers to a Python-only implementation; maybe that is another option.