I see this in the python source code being tested (datetimetester.py), so I guess it is a rounding problem : 

# maximum timestamp: set seconds to zero to avoid rounding issues
        max_dt = self.theclass.max.replace(tzinfo=timezone.utc,
                                           second=0, microsecond=0)
        max_ts = max_dt.timestamp()
        # date 9999-12-31 23:59:00+00:00: timestamp 253402300740
        self.assertEqual(self.theclass.fromtimestamp(max_ts, tz=timezone.utc),
                         max_dt)