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/2481#issuecomment-429497014,https://api.github.com/repos/pydata/xarray/issues/2481,429497014,MDEyOklzc3VlQ29tbWVudDQyOTQ5NzAxNA==,6628425,2018-10-13T00:42:35Z,2018-10-13T00:42:35Z,MEMBER,"I think this would be a fair amount of work to implement :), but in principle it would be a natural step forward in the pattern we have been following in porting time series functionality from pandas for use with calendars supported by `cftime`.  

In the short term, at least for the use-case you describe above, I think a potentially simpler option would be to use a call to `CFTimeIndex.shift`; based on your example I'll illustrate with the DatetimeIndex version, since `resample` doesn't exist yet for a CFTimeIndex:
```
In [1]: import xarray as xr; import pandas as pd

In [2]: times = pd.date_range('2000', periods=361)

In [3]: da = xr.DataArray(range(361), [('time', times)])

In [4]: monthly_count = da.resample(time='M').count()

In [5]: end_time = monthly_count.indexes['time']

In [6]: start_time = end_time.shift(-1, 'M')

In [7]: expected_days_in_group = end_time - start_time

In [8]: expected_days_in_group
Out[8]:
TimedeltaIndex(['31 days', '29 days', '31 days', '30 days', '31 days',
                '30 days', '31 days', '31 days', '30 days', '31 days',
                '30 days', '31 days'],
               dtype='timedelta64[ns]', name=u'time', freq=None)
```
Note that currently subtracting one CFTimeIndex from another (analogous to `end_time - start_time` in the example above) raises an error (I think it should also return a TimedeltaIndex), but that's probably a straightforward fix (I'll create a separate issue):
```
In [8]: a = xr.cftime_range('2000', periods=12, freq='M')

In [9]: b = a.shift(-1, 'M')

In [10]: a - b
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-10-09bd029d0285> in <module>()
----> 1 a - b

/Users/spencerclark/xarray-dev/xarray/xarray/coding/cftimeindex.pyc in __sub__(self, other)
    365
    366     def __sub__(self, other):
--> 367         return CFTimeIndex(np.array(self) - other)
    368
    369

TypeError: unsupported operand type(s) for -: 'numpy.ndarray' and 'CFTimeIndex'
```

-----

As a side note, support even for standard PeriodIndexes in xarray needs some work -- see the following existing open issues: #1270, #1565.  Serialization to netCDF files would also be nice to support (but I don't see an existing issue for that).  I'm sure work to fix these issues would also be appreciated!","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,369639339