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.

","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,558293655
https://github.com/pydata/xarray/issues/3739#issuecomment-580860461,https://api.github.com/repos/pydata/xarray/issues/3739,580860461,MDEyOklzc3VlQ29tbWVudDU4MDg2MDQ2MQ==,1197350,2020-01-31T18:46:19Z,2020-01-31T18:46:19Z,MEMBER,"Hi @avatar101 - thanks for your issue!

I couldn't fully reproduce your example, since it references files that I don't have access to. For the future, please consider trying to create a ""Minimal, Complete and Verifiable Example"" (MCVE): (http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports)

In the meantime, I do have a suggestion you could try. Instead of
```python
ds_test.time.attrs['units'] = ""Seconds since 1970-01-01 00:00:00""
```
try
```python
ds_test.time.encoding['units'] = ""Seconds since 1970-01-01 00:00:00""
```

The reason is that, since you have created your time coordinate using `pd.date_range`, which returns a `datetime64` dtype, that variable is already considered to be encoded.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,558293655