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
965072308,MDU6SXNzdWU5NjUwNzIzMDg=,5687,Make cftime dateoffsets public,2448579,open,0,,,2,2021-08-10T14:57:39Z,2021-08-10T23:28:20Z,,MEMBER,,,,"Consider the following cftime vector. It's fairly common to see users asking how to subtract ""1 month"" from this kind of vector:

``` python
xr.set_options(display_style=""text"")
time = xr.DataArray(
    xr.cftime_range(""1000-01-01"", ""1000-05-01"", freq=""MS"", calendar=""360_day""), dims=""time"", name=""time""
)
time
```
```
<xarray.DataArray 'time' (time: 5)>
array([cftime.Datetime360Day(1000, 1, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 2, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 3, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 4, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 5, 1, 0, 0, 0, 0, has_year_zero=False)],
      dtype=object)
Coordinates:
  * time     (time) object 1000-01-01 00:00:00 ... 1000-05-01 00:00:00
```

Subtracting `pd.Timedelta(""1 month"")` does not work because a month does not represent an absolute unit of time. Instead the solution appears to be:
``` python
time - xr.coding.cftime_offsets.MonthBegin(1)
```
```
<xarray.DataArray 'time' (time: 5)>
array([cftime.Datetime360Day(999, 12, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 1, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 2, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 3, 1, 0, 0, 0, 0, has_year_zero=False),
       cftime.Datetime360Day(1000, 4, 1, 0, 0, 0, 0, has_year_zero=False)],
      dtype=object)
Coordinates:
  * time     (time) object 1000-01-01 00:00:00 ... 1000-05-01 00:00:00
```

I think pandas exposes this functionality as `pd.DateOffset(months=1)`. Can we add a similar `xr.DateOffset`?
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