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
1498255931,I_kwDOAMm_X85ZTZI7,7381,Document cftime arithmetic limitations,5659125,open,0,,,3,2022-12-15T11:24:04Z,2023-07-17T09:34:12Z,,NONE,,,,"### What is your issue?

Hey all,
i have quick use case where i am unsure if that's related to xarray. I just want to do simple arithmetics with a cftime time axis, e.g.

```python
import xarray as xr
import numpy as np
time = xr.DataArray(xr.cftime_range(start=""2000"", periods=6, freq=""1MS"", calendar=""noleap""), dims='time')
```
and then, e.g., compute mid of those timesteps:
```python
time[:-1] + 0.5 * time.diff('time')
```
which results in
```
---------------------------------------------------------------------------
UFuncTypeError                            Traceback (most recent call last)
Input In [72], in <cell line: 1>()
----> 1 time[1:] + 0.5 * time.diff('time')

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/_typed_ops.py:206, in DataArrayOpsMixin.__add__(self, other)
    205 def __add__(self, other):
--> 206     return self._binary_op(other, operator.add)

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/dataarray.py:3530, in DataArray._binary_op(self, other, f, reflexive)
   3526 other_variable = getattr(other, ""variable"", other)
   3527 other_coords = getattr(other, ""coords"", None)
   3529 variable = (
-> 3530     f(self.variable, other_variable)
   3531     if not reflexive
   3532     else f(other_variable, self.variable)
   3533 )
   3534 coords, indexes = self.coords._merge_raw(other_coords, reflexive)
   3535 name = self._result_name(other)

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/_typed_ops.py:396, in VariableOpsMixin.__add__(self, other)
    395 def __add__(self, other):
--> 396     return self._binary_op(other, operator.add)

File /work/ch0636/g300046/conda_envs/pyremo-dev/lib/python3.9/site-packages/xarray/core/variable.py:2519, in Variable._binary_op(self, other, f, reflexive)
   2516 attrs = self._attrs if keep_attrs else None
   2517 with np.errstate(all=""ignore""):
   2518     new_data = (
-> 2519         f(self_data, other_data) if not reflexive else f(other_data, self_data)
   2520     )
   2521 result = Variable(dims, new_data, attrs=attrs)
   2522 return result

UFuncTypeError: ufunc 'add' cannot use operands with types dtype('O') and dtype('<m8[ns]')
```
The problem is the differentiation results in `np.timedelta64` objects:
```python
time.diff('time')
```
![grafik](https://user-images.githubusercontent.com/5659125/207844626-76adf571-92f7-4645-9e9d-5ba01d08afa3.png)
with which the arithmetics are not implemented in `cftime`. However, differentiation with `datetime.timdelta` (which is wrapped by `np.timedelta64`) works in this case since it's implemented in cftime, e.g., when i do `np.diff`, it works:
```python
time[:-1] + 0.5 * np.diff(time)
```
![grafik](https://user-images.githubusercontent.com/5659125/207847974-c263ae12-142e-4004-bcdf-847379b3dc75.png)

i get the expected result. I am not sure what to expect from xarray and if this might be an issue of cftime (e.g., implement differentiation with `np.timedelta64`). I was wondering why the differentiation with cftime objects results in `np.timedelta64` and not `datetime.timedelta` objects (like in the pure numpy differentation).
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/7381/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue
1507705136,I_kwDOAMm_X85Z3cEw,7398,convert_calendar adds time coordinate to static variables,5659125,open,0,,,1,2022-12-22T11:27:11Z,2023-01-15T15:51:21Z,,NONE,,,,"### What is your issue?

I just recognized that `convert_calendar` adds a time coordinate to variables that have been static before, e.g.,

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

np.random.seed(0)
temperature = 15 + 8 * np.random.randn(2, 2, 3)
precipitation = 10 * np.random.rand(2, 2, 3)
orog = 0 + 100 * np.random.randn(2, 2)
lon = [[-99.83, -99.32], [-99.79, -99.23]]
lat = [[42.25, 42.21], [42.63, 42.59]]
time = pd.date_range(""2014-09-06"", periods=3)
reference_time = pd.Timestamp(""2014-09-05"")


ds = xr.Dataset(
    data_vars=dict(
        temperature=([""x"", ""y"", ""time""], temperature),
        precipitation=([""x"", ""y"", ""time""], precipitation),
        orog = ([""x"", ""y""], orog)
    ),
    coords=dict(
        lon=([""x"", ""y""], lon),
        lat=([""x"", ""y""], lat),
        time=time,
        reference_time=reference_time,
    ),
    attrs=dict(description=""Weather related data.""),
)
ds
```
![grafik](https://user-images.githubusercontent.com/5659125/209124247-4d6bb64f-8396-4a90-bcce-c420b022b0cf.png)
I would like to convert the time axis to cftime objects using
```python
ds.convert_calendar(ds.time.dt.calendar, use_cftime=True)
ds
```
![grafik](https://user-images.githubusercontent.com/5659125/209124475-2ca60508-2d39-432b-816d-0f2718cda5b4.png)
However, this results in an unexpected time coordinate in the `orog` variable...","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/7398/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue