Period.end_time is same as start_time when period is D, B, H, T, or S
For example:

> > > p = pandas.Period('2012-10-23', 'D')
> > > p.start_time
> > > <Timestamp: 2012-10-23 00:00:00>
> > > p.end_time
> > > <Timestamp: 2012-10-23 00:00:00>

There are tests, but they fail to detect the problem because they are comparing the end_time to a constant which is the start_time (so they pass).
