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
681279877,MDU6SXNzdWU2ODEyNzk4Nzc=,4347,to_zarr() failing after concatenating netcdfs with different time indexes,23692810,open,0,,,5,2020-08-18T19:30:30Z,2022-04-28T15:09:10Z,,NONE,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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**What happened**:
After concatenating two NetCDF DataSets with different cftime.DateTimeNoLeap coordinates, attempting to write to a Zarr store with `ds.to_zarr()` fails with an OutOfBoundsDatetime exception.

**What you expected to happen**:
I expect `to_zarr()` to execute successfully.

**Minimal Complete Verifiable Example**:

```python
import xarray as xr
import cftime
import pandas as pd

# open a generic CESM dataset containing a time_bnds variable
url = 'http://adss.apcc21.org/opendap/CMIP5DB/cmip5_daily_BT/pr_day_CESM1-BGC_rcp85_r1i1p1_20760101-21001231.nc'
ds = xr.open_dataset(url)

# create two new DataSets with different, overlapping time indexes.
ds2 = ds.sel(time=slice(None, cftime.DatetimeNoLeap(2076, 3, 1, 1, 0, 0, 0)))
ds3 = ds.sel(time=slice(None, cftime.DatetimeNoLeap(2076, 2, 1, 1, 0, 0, 0)))

# concatenate the two DataSets, using the default fillvalue
ds4 = xr.concat([ds2, ds3], dim=pd.Index(['ds2','ds3'], name='ds'))

# fails with OutOfBoundsDatetime exception
zs = ds4.to_zarr('/tmp/my_zarr.zarr')

```

**Anything else we need to know?**:
I believe the problem is related to the implicit NaN fillvalue used in concatenating the `time_bnds` variable. I arrived at the code while trying to produce a minimal example of a similar error, where a DataSet concatenation would fail with a SerializationError if I tried to concatenate multiple datasets containing `time_bnds` variables. In the above example and in my earlier troubleshooting in production code, removing `time_bnds` with `ds = ds.drop('time_bnds')` made the `to_zarr()` command work.

This is a common use case since `time_bnds` indexes are generated by CESM climate model output. 

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.7 (default, Mar 23 2020, 22:36:06) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 5.3.0-1032-aws
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.1

xarray: 0.16.0
pandas: 1.0.5
numpy: 1.19.1
scipy: 1.5.0
netCDF4: 1.4.2
pydap: None
h5netcdf: 0.8.0
h5py: 2.10.0
Nio: None
zarr: 2.3.2
cftime: 1.2.1
nc_time_axis: 1.2.0
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.20.0
distributed: 2.20.0
matplotlib: 3.2.2
cartopy: 0.17.0
seaborn: None
numbagg: None
pint: None
setuptools: 49.2.0.post20200714
pip: 20.1.1
conda: None
pytest: None
IPython: 7.16.1
sphinx: None
/home/ubuntu/a


</details>
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