Subtracting Series of Timestamps gives wrong result
Example:

In [11]: d0 = pd.to_datetime(pd.Series(["12/1/2011"]))

In [12]: d1 = pd.to_datetime(pd.Series(["12/3/2011"]))

In [13]: d1[0] - d0[0]
Out[13]: datetime.timedelta(2)

In [14]: (d1 - d0)[0]
Out[14]: 2000 days, 0:00:00
