I'm not sure if I'm able to reopen #591 - seems I'm not.
However, I got the same (similar) error today, with most recent pyjamas sources.
_dto = (self.date_to.getDate()+timedelta(days=32)).replace(day=1)
And got such traceback:
maho@dlaptop:~/[...]py$ bash build.sh
Original issue: http://code.google.com/p/pyjamas/issues/detail?id=679 (January 28, 2012 18:33:27)