What kinds of illegal date values are you trying to represent? The reason I ask is that it's not going to be as easy as subclassing datetime... datetime is implemented in C. and so enforcement of legal values is going to be in the C code. For the time.h functions, you're also going to be constrained by the size of the time_t struct, which is probably a long int on your platform. See Modules/datetimemodule.c in the Python source.
One thing you could do would be to copy datetimemodule.c and build your own C extension type based on it... things like MAXYEAR 9999 could be changed that way. The other thing would be to write a pure-python datetime class without trying to inherit datetime.datetime. -- http://mail.python.org/mailman/listinfo/python-list
