> the time_t OverflowError

The issue here is that for a large date, dt.timestamp() returns a float large enough to cause overflow in fromtimestamp.

> Let me know if you want any more info.

Can you figure out what date causes this (0002-01-01 or 9998-12-12 or both)?  What value is returned by dt.timestamp()?  Does pure python implementation behave the same as C? (SEt sys.modules['_datetime'] to None before importing datetime to get a pure python implementation.)

> In my case the message for the test_all() failure is “posix/Africa/Casablanca system_transitions”.

Do you get this failure only on a 32-bit interpreter?  Please add

class CasablancaTest(ZoneInfoTest):
    zonename = 'Africa/Casablanca'

to datetimetester.py and run python -mtest -v test_datetime.