id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
322091500,MDU6SXNzdWUzMjIwOTE1MDA=,2116,time array with mixture of types decoded from non-standard calendar ,11436996,closed,0,,,2,2018-05-10T21:27:52Z,2018-05-10T22:04:15Z,2018-05-10T22:04:15Z,NONE,,,,"Reading a set of files with 360-day calendar with open_mf_dataset(), yields a time array with a mixture of types cftime._cftime.Datetime360Day and Timestamp. The files are CMIP5 output for HadGEM2-ES. I don't think there is a way to force it to use only cftime type for all times? All times should have the same type so slicing is possible (see below).

```python
precip = xy.open_mfdataset(dataDir+'prc_day_HadGEM2-ES_rcp85_r1i1p1_*.nc')
precip['prc'].time 
```
outputs:
```<xarray.DataArray 'time' (time: 33870)>
array([cftime._cftime.Datetime360Day(2005, 12, 1, 12, 0, 0, 0, -1, 331),
       cftime._cftime.Datetime360Day(2005, 12, 2, 12, 0, 0, 0, -1, 332),
       cftime._cftime.Datetime360Day(2005, 12, 3, 12, 0, 0, 0, -1, 333), ...,
       Timestamp('2099-12-28 12:00:00'), Timestamp('2099-12-29 12:00:00'),
       Timestamp('2099-12-30 12:00:00')], dtype=object)
Coordinates:
  * time     (time) object 2005-12-01 12:00:00 2005-12-02 12:00:00 ...
Attributes:
    standard_name:  time
    long_name:      time
    bounds:         time_bnds
    axis:           T
```

#### Problem description
One consistent type is needed for slicing. It would be fine if they were all type cftime.Datetime360Day, as one could then do the following:
```python
startslice = cftime.Datetime360Day(startYear, 1, 1,0,0,0,0,-1,1)
endslice = cftime.Datetime360Day(endYear, 12, 30,23,0,0,0,-1,360)
precip.sel(time=slice(startslice,endslice))
```
but with the mixture of types, this returns a KeyError

#### Expected Output
For some reason with another set of 360-day calendar files (from HadGEM2-CC for what it's worth) it did decode for me with all one type, the time array looks as expected, and slicing as above works:
```<xarray.DataArray 'time' (time: 34230)>
array([cftime._cftime.Datetime360Day(2005, 12, 1, 12, 0, 0, 0, -1, 331),
       cftime._cftime.Datetime360Day(2005, 12, 2, 12, 0, 0, 0, -1, 332),
       cftime._cftime.Datetime360Day(2005, 12, 3, 12, 0, 0, 0, -1, 333), ...,
       cftime._cftime.Datetime360Day(2100, 12, 28, 12, 0, 0, 0, -1, 358),
       cftime._cftime.Datetime360Day(2100, 12, 29, 12, 0, 0, 0, -1, 359),
       cftime._cftime.Datetime360Day(2100, 12, 30, 12, 0, 0, 0, -1, 360)],
      dtype=object)
Coordinates:
  * time     (time) object 2005-12-01 12:00:00 2005-12-02 12:00:00 ...
Attributes:
    standard_name:  time
    long_name:      time
    bounds:         time_bnds
    axis:           T
```

#### Output of ``xr.show_versions()``
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.15.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-693.21.1.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

xarray: 0.10.3
pandas: 0.22.0
numpy: 1.14.3
scipy: 1.1.0
netCDF4: 1.4.0
h5netcdf: 0.5.1
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: 1.0.0
dask: 0.17.4
distributed: 1.21.8
matplotlib: 2.2.2
cartopy: 0.16.0
seaborn: 0.8.1
setuptools: 39.1.0
pip: 9.0.3
conda: 4.5.3
pytest: None
IPython: 5.6.0
sphinx: None","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2116/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue