I try to remove seconds from date:
>>> import datetime
>>> test1 = datetime.datetime(2011, 6, 10, 0, 0)
>>> test1
datetime.datetim
rstrip takes a set (although the argument can be any iterable, like str in your example) of characters that are removed, not a single string.
And by the way, the string representation of datetime.datetime is not fixed, you can't rely on it. Instead, use isoformat on the date or strftime:
>>> import datetime
>>> test1 = datetime.datetime(2011, 6, 10, 0, 0)
>>> test1.date().isoformat()
'2011-06-10'
>>> test1.strftime('%Y-%m-%d')
'2011-06-10'