BUG: Series flex methods don't work with datetime-likes
It's because of Series._bin_op says @jreback.

This fails.

``` python
>>> from datetime import timedelta
>>> from pandas import Series, DataFrame, date_range
>>> s  = Series(date_range('2012-1-1', periods=3, freq='D'))
>>> s.sub(s.max())
TypeError: ufunc sub cannot use operands with types dtype('<M8[ns]') and dtype('O')
```

Probably the same thing for all the valid time operations.
