html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/3535#issuecomment-554648042,https://api.github.com/repos/pydata/xarray/issues/3535,554648042,MDEyOklzc3VlQ29tbWVudDU1NDY0ODA0Mg==,6628425,2019-11-16T15:39:23Z,2019-11-16T15:39:23Z,MEMBER,"Thanks for raising this issue @mathause.  In hindsight this does not surprise me.  Pandas's strict use of nanosecond-resolution datetimes and timedeltas was part of the motivation for the `CFTimeIndex`.  While convenient, because it allows us to re-use code already written in pandas, holding the result of the difference between two CFTimeIndexes in a `TimedeltaIndex` clearly prevents us from taking the difference between distant dates.  

Perhaps a more robust (yet more complex) solution for https://github.com/pydata/xarray/issues/2484 would be to write a version of a `TimedeltaIndex` that does not internally cast the timedeltas to type `np.timedelta64[ns]`, and rather leaves them as `datetime.timedelta` objects, which are the actual result of subtracting two sequences of `cftime.datetime` objects.

Regarding the `combine_by_coords` issue, though, there might be an easier fix.  Is there a reason that `first_items` is an `Index` of length-one Indexes?  It's not clear to me why that needs to be the case.

https://github.com/pydata/xarray/blob/56c16e4bf45a3771fd9acba76d802c0199c14519/xarray/core/combine.py#L91

It appears if we just select the first value of each index (i.e. a `cftime.datetime` object in this example), e.g.

```python
first_items = pd.Index([index[0] for index in indexes])
```
pandas's `rank` method works properly and `combine_by_coords` produces the correct result:

```
>>> xr.combine_by_coords([d1, d2, d3]).time
<xarray.DataArray 'time' (time: 3)>
array([cftime.DatetimeGregorian(4500, 12, 31, 0, 0, 0, 0, 4, 365),
       cftime.DatetimeGregorian(4600, 12, 31, 0, 0, 0, 0, 2, 365),
       cftime.DatetimeGregorian(5100, 12, 31, 0, 0, 0, 0, 0, 365)],
      dtype=object)
Coordinates:
  * time     (time) object 4500-12-31 00:00:00 ... 5100-12-31 00:00:00
```","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,523037716