issues: 327392061

Code Sample, a copy-pastable example if possible

```python import numpy as np import pandas as pd import xarray as xr

time = np.arange('2005-02-01', '2007-03-01', dtype='datetime64') arr = xr.DataArray( np.arange(time.size), coords=[time,], dims=('time',), name='data' ) arr.resample(time='M').interpolate('linear')

---

ValueError Traceback (most recent call last) <ipython-input-7-6a92b6afe08e> in <module>() 7 np.arange(time.size), coords=[time,], dims=('time',), name='data' 8 ) ----> 9 arr.resample(time='M').interpolate('linear')

~/devel/c3s-cns/venv_op/lib/python3.6/site-packages/xarray/core/resample.py in interpolate(self, kind) 108 109 """ --> 110 return self._interpolate(kind=kind) 111 112 def _interpolate(self, kind='linear'):

~/devel/c3s-cns/venv_op/lib/python3.6/site-packages/xarray/core/resample.py in _interpolate(self, kind) 218 elif self._dim not in v.dims: 219 coords[k] = v --> 220 return DataArray(f(new_x), coords, dims, name=dummy.name, 221 attrs=dummy.attrs) 222

~/devel/c3s-cns/venv_op/lib/python3.6/site-packages/scipy/interpolate/polyint.py in call(self, x) 77 """ 78 x, x_shape = self._prepare_x(x) ---> 79 y = self._evaluate(x) 80 return self._finish_y(y, x_shape) 81

~/devel/c3s-cns/venv_op/lib/python3.6/site-packages/scipy/interpolate/interpolate.py in _evaluate(self, x_new) 632 y_new = self._call(self, x_new) 633 if not self._extrapolate: --> 634 below_bounds, above_bounds = self._check_bounds(x_new) 635 if len(y_new) > 0: 636 # Note fill_value must be broadcast up to the proper size

~/devel/c3s-cns/venv_op/lib/python3.6/site-packages/scipy/interpolate/interpolate.py in _check_bounds(self, x_new) 664 "range.") 665 if self.bounds_error and above_bounds.any(): --> 666 raise ValueError("A value in x_new is above the interpolation " 667 "range.") 668

ValueError: A value in x_new is above the interpolation range. ```

Problem description

The internal format of arr.time is datetime64[D]

```python arr.time

<xarray.DataArray 'time' (time: 758)> array(['2005-02-01', '2005-02-02', '2005-02-03', ..., '2007-02-26', '2007-02-27', '2007-02-28'], dtype='datetime64[D]') Coordinates: * time (time) datetime64[D] 2005-02-01 2005-02-02 2005-02-03 ... ``` Internally there is a cast to float, for both the old time indices x and the new time indices new_x, but the new time indices are in datetime64[ns], so they don't match.

DataArrayResample._interpolate

```python x = self._obj[self._dim].astype('float') y = self._obj.data

   axis = self._obj.get_axis_num(self._dim)

   f = interp1d(x, y, kind=kind, axis=axis, bounds_error=True,
                assume_sorted=True)
   new_x = self._full_index.values.astype('float')

``` With a cast to datetime64[ns] it works:

```python import numpy as np import pandas as pd import xarray as xr

time = np.arange('2005-02-01', '2007-03-01', dtype='datetime64').astype('datetime64[ns]') arr = xr.DataArray( np.arange(time.size), coords=[time,], dims=('time',), name='data' ) arr.resample(time='M').interpolate('linear') <xarray.DataArray 'data' (time: 25)> array([ 27., 58., 88., 119., 149., 180., 211., 241., 272., 302., 333., 364., 392., 423., 453., 484., 514., 545., 576., 606., 637., 667., 698., 729., 757.]) Coordinates: * time (time) datetime64[ns] 2005-02-28 2005-03-31 2005-04-30 ... ```

Expected Output

python <xarray.DataArray 'data' (time: 25)> array([ 27., 58., 88., 119., 149., 180., 211., 241., 272., 302., 333., 364., 392., 423., 453., 484., 514., 545., 576., 606., 637., 667., 698., 729., 757.]) Coordinates: * time (time) datetime64[ns] 2005-02-28 2005-03-31 2005-04-30 ...

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.0.final.0 python-bits: 64 OS: Linux OS-release: 4.13.0-43-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8 xarray: 0.10.4 pandas: 0.20.3 numpy: 1.13.1 scipy: 1.0.0 netCDF4: 1.3.1 h5netcdf: None h5py: None Nio: None zarr: None bottleneck: None cyordereddict: None dask: 0.16.1 distributed: None matplotlib: 2.0.2 cartopy: None seaborn: None setuptools: 38.4.0 pip: 10.0.1 conda: None pytest: 3.4.0 IPython: 6.1.0 sphinx: None