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
398152613,MDU6SXNzdWUzOTgxNTI2MTM=,2667,datetime interpolation doesn't work,2448579,closed,0,,,4,2019-01-11T06:45:55Z,2019-02-11T09:47:09Z,2019-02-11T09:47:09Z,MEMBER,,,,"#### Code Sample, a copy-pastable example if possible

This code doesn't work anymore on master.

```python
a = xr.DataArray(np.arange(21).reshape(3, 7), 
                 dims=['x', 'time'], 
                 coords={'x': [1, 2, 3], 'time': pd.date_range('01-01-2001', periods=7, freq='D')})
xi = xr.DataArray(np.linspace(1, 3, 50),  
                  dims=['time'], 
                  coords={'time': pd.date_range('01-01-2001', periods=50, freq='H')})
a.interp(x=xi, time=xi.time)
```

#### Problem description

The above code now raises the error

```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-26-dda3a6d5725b> in <module>
      6                   dims=['time'],
      7                   coords={'time': pd.date_range('01-01-2001', periods=50, freq='H')})
----> 8 a.interp(x=xi, time=xi.time)

~/work/python/xarray/xarray/core/dataarray.py in interp(self, coords, method, assume_sorted, kwargs, **coords_kwargs)
   1032         ds = self._to_temp_dataset().interp(
   1033             coords, method=method, kwargs=kwargs, assume_sorted=assume_sorted,
-> 1034             **coords_kwargs)
   1035         return self._from_temp_dataset(ds)
   1036 

~/work/python/xarray/xarray/core/dataset.py in interp(self, coords, method, assume_sorted, kwargs, **coords_kwargs)
   2008                         in indexers.items() if k in var.dims}
   2009                     variables[name] = missing.interp(
-> 2010                         var, var_indexers, method, **kwargs)
   2011                 elif all(d not in indexers for d in var.dims):
   2012                     # keep unrelated object array

~/work/python/xarray/xarray/core/missing.py in interp(var, indexes_coords, method, **kwargs)
    468     new_dims = broadcast_dims + list(destination[0].dims)
    469     interped = interp_func(var.transpose(*original_dims).data,
--> 470                            x, destination, method, kwargs)
    471 
    472     result = Variable(new_dims, interped, attrs=var.attrs)

~/work/python/xarray/xarray/core/missing.py in interp_func(var, x, new_x, method, kwargs)
    535                              new_axis=new_axis, drop_axis=drop_axis)
    536 
--> 537     return _interpnd(var, x, new_x, func, kwargs)
    538 
    539 

~/work/python/xarray/xarray/core/missing.py in _interpnd(var, x, new_x, func, kwargs)
    558     var = var.transpose(range(-len(x), var.ndim - len(x)))
    559     # stack new_x to 1 vector, with reshape
--> 560     xi = np.stack([x1.values.ravel() for x1 in new_x], axis=-1)
    561     rslt = func(x, var, xi, **kwargs)
    562     # move back the interpolation axes to the last position

~/work/python/xarray/xarray/core/missing.py in <listcomp>(.0)
    558     var = var.transpose(range(-len(x), var.ndim - len(x)))
    559     # stack new_x to 1 vector, with reshape
--> 560     xi = np.stack([x1.values.ravel() for x1 in new_x], axis=-1)
    561     rslt = func(x, var, xi, **kwargs)
    562     # move back the interpolation axes to the last position

AttributeError: 'numpy.ndarray' object has no attribute 'values'
```

I think the issue is this line which returns a numpy array instead of a Variable. This was added in the `coarsen` PR (cc @fujiisoup)
https://github.com/pydata/xarray/blob/d4c46829b283ab7e7b7db8b86dae77861ce68f3c/xarray/core/utils.py#L636","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2667/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue