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/3739#issuecomment-586978341,https://api.github.com/repos/pydata/xarray/issues/3739,586978341,MDEyOklzc3VlQ29tbWVudDU4Njk3ODM0MQ==,10194086,2020-02-17T12:48:55Z,2020-02-17T12:48:55Z,MEMBER,"The following works (using `assign_coords()` instead of `time.values=`). I think you also forgot a `str` in `date_range`.

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

files = ['ds_1.nc', 'ds_2.nc']

ds_test = xr.open_mfdataset(files, combine='nested', concat_dim='time', decode_cf=False)
yr = 1988 # year
dates = pd.date_range(start=str(yr), end=str(yr+1), freq='6H', closed='left')
ds_test.assign_coords(time=dates[:2])
ds_test.time.encoding['units'] = ""seconds since 1970-01-01 00:00:00""
ds_test.time.encoding['calendar'] = ""proleptic_gregorian""
ds_test.to_netcdf('ds_1_2.nc')

```

The following works as well:
``` python
ds_test = xr.open_mfdataset(files, combine='nested', concat_dim='time')
ds_test.time.encoding['units'] = ""seconds since 1970-01-01 00:00:00""
ds_test.to_netcdf('ds_1_2.nc')
```

---

However, what does indeed not work is the following

``` python
ds_test = xr.open_mfdataset(files, combine='nested', concat_dim='time')
ds_test.time.attrs['units'] = ""seconds since 1970-01-01 00:00:00""
ds_test.to_netcdf('ds_1_2.nc')
```
which I don't entirely understand, because `ds_test.time.encoding` is empty. So maybe there is an `encoding` hidden somewhere, but I couldn't find it.

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