issues: 398918281

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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
398918281 MDU6SXNzdWUzOTg5MTgyODE= 2671 Enable subtracting a scalar cftime.datetime object from a CFTimeIndex 6628425 closed 0     0 2019-01-14T14:42:12Z 2019-01-30T16:45:10Z 2019-01-30T16:45:10Z MEMBER      

Code Sample, a copy-pastable example if possible

``` In [1]: import xarray

In [2]: times = xarray.cftime_range('2000', periods=3)

In [3]: times - times[0]

TypeError Traceback (most recent call last) <ipython-input-3-97cbca76a8af> in <module> ----> 1 times - times[0]

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in sub(self, other) 417 return CFTimeIndex(np.array(self) - other.to_pytimedelta()) 418 else: --> 419 return CFTimeIndex(np.array(self) - other) 420 421 def _add_delta(self, deltas):

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in new(cls, data, name) 238 result = object.new(cls) 239 result._data = np.array(data, dtype='O') --> 240 assert_all_valid_date_type(result._data) 241 result.name = name 242 return result

~/xarray-dev/xarray/xarray/coding/cftimeindex.py in assert_all_valid_date_type(data) 194 raise TypeError( 195 'CFTimeIndex requires cftime.datetime ' --> 196 'objects. Got object of {}.'.format(date_type)) 197 if not all(isinstance(value, date_type) for value in data): 198 raise TypeError(

TypeError: CFTimeIndex requires cftime.datetime objects. Got object of <class 'datetime.timedelta'>. ```

Problem description

This should result in a pandas.TimedeltaIndex, as is the case for a pandas.DatetimeIndex: ``` In [4]: import pandas

In [5]: times = pandas.date_range('2000', periods=3)

In [6]: times - times[0] Out[6]: TimedeltaIndex(['0 days', '1 days', '2 days'], dtype='timedelta64[ns]', freq=None) ```

Expected Output

``` In [1]: import xarray

In [2]: times = xarray.cftime_range('2000', periods=3)

In [3]: times - times[0] Out[3]: TimedeltaIndex(['0 days', '1 days', '2 days'], dtype='timedelta64[ns]', freq=None) ```

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.6 | packaged by conda-forge | (default, Jul 26 2018, 09:55:02) [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)] python-bits: 64 OS: Darwin OS-release: 18.2.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.2 xarray: 0.10.9+127.ga7129d1 pandas: 0.24.0.dev0+1332.g5d134ec numpy: 1.15.4 scipy: 1.1.0 netCDF4: 1.4.2 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.0.3.4 PseudonetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None cyordereddict: None dask: 1.0.0 distributed: 1.25.1 matplotlib: 3.0.2 cartopy: None seaborn: 0.9.0 setuptools: 40.6.3 pip: 18.1 conda: None pytest: 3.10.1 IPython: 7.2.0 sphinx: None 
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