issue_comments: 328637548

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https://github.com/pydata/xarray/issues/1565#issuecomment-328637548 https://api.github.com/repos/pydata/xarray/issues/1565 328637548 MDEyOklzc3VlQ29tbWVudDMyODYzNzU0OA== 10050469 2017-09-11T19:43:19Z 2017-09-11T19:43:19Z MEMBER

OK, so the problem is that when given to is_datetime_like here, the pd.PeriodIndex is already transformed into an array of dtype object.

Before @darothen 's https://github.com/pydata/xarray/pull/1356, the piece if code that was executed is date = ref_var.to_index(), which has the nice property to return a PeriodIndex, as shown here from my debugger:

```python

ref_var Out[1]: <xarray.IndexVariable 'time' (time: 12)> array([Period('2000-01', 'M'), Period('2000-02', 'M'), Period('2000-03', 'M'), Period('2000-04', 'M'), Period('2000-05', 'M'), Period('2000-06', 'M'), Period('2000-07', 'M'), Period('2000-08', 'M'), Period('2000-09', 'M'), Period('2000-10', 'M'), Period('2000-11', 'M'), Period('2000-12', 'M')], dtype=object) ref_var.to_index() Out[3]: PeriodIndex(['2000-01', '2000-02', '2000-03', '2000-04', '2000-05', '2000-06', '2000-07', '2000-08', '2000-09', '2000-10', '2000-11', '2000-12'], dtype='period[M]', name='time', freq='M') ```

 
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