correct test_timestamp

The date being tested is (2010, 9, 21) is indeed equivalent to
1285041600 Unixtime:

```
>>> import time, datetime
>>> d = datetime.datetime.strptime('2010-09-21 00:00:00', '%Y-%m-%d %H:%M:%S')
>>> int(time.mktime(d.timetuple()))
1285041600
```

Making this change breaks no other tests.