BUG: Wrong order and ambiguous start point  for some period values
```python
>>>pd.period_range('1350-01-01', '1350-01-02', freq='6h').sort_values()
PeriodIndex(['1350-01-01 23:00', '1350-01-01 05:00', '1350-01-01 11:00',
             '1350-01-01 17:00', '1350-01-02 23:00'],
            dtype='period[6H]', freq='6H')
```
Though sorted, '1350-01-01 23:00' still locates the first.
```python
>>>pd.period_range('2150-01-01', '2150-01-02', freq='6h')
PeriodIndex(['2150-01-01 00:00', '2150-01-01 06:00', '2150-01-01 12:00',
             '2150-01-01 18:00', '2150-01-02 00:00'],
            dtype='period[6H]', freq='6H')
```
And why this starts at 00:00, but the former one starts at 23:00?

Thanks!