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-590115864,https://api.github.com/repos/pydata/xarray/issues/3739,590115864,MDEyOklzc3VlQ29tbWVudDU5MDExNTg2NA==,33062222,2020-02-23T21:03:53Z,2020-02-23T21:03:53Z,NONE,"@mathause 

thanks for your suggestions. Your first solution works fine for correcting the time data stored in the array. I also don't understand why `ds_test.time.encoding` is empty and yet, its the reason for an error while saving it. Maybe its a bug?

@Chan-Jer  Another work around which I used was to set the correct time value using cdo's `settime` function.

","{""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-580935355,https://api.github.com/repos/pydata/xarray/issues/3739,580935355,MDEyOklzc3VlQ29tbWVudDU4MDkzNTM1NQ==,33062222,2020-01-31T22:17:34Z,2020-01-31T22:17:34Z,NONE,"Hi Ryan, thanks for your reply. Apologies for not creating a reproducible problem earlier as the files weren't created by xarray routine. Please find my attempt at reproducing the problem below:

## Minimum steps to reproduce the error
```python
import numpy as np
import xarray as xr
import pandas as pd

data1 = np.ones(shape=(1, 181, 360))
lats=np.arange(-90,91, 1)
lons=np.arange(-180,180,1)
time1 = np.array([0])

# creating the first dataset
da_1 = xr.DataArray(data1, coords=[time1, lats, lons], dims=['time', 'lats', 'lons'])
da_1.time.attrs['units'] = ""hours since 1988-01-01 00:00:00""
da_1.time.attrs['calendar'] = ""proleptic_gregorian""
da_1.time.attrs['standard_name'] = ""time""
ds_1 = xr.Dataset({'V':da_1})
ds_1.attrs['Conventions'] = 'CF'
ds_1.to_netcdf('ds_1.nc', encoding=None)

# creating second test dataset
time2=np.array([6])  # wrong time value
da_2 = xr.DataArray(data1, coords=[time2, lats, lons], dims=['time', 'lats', 'lons'])
da_2.time.attrs['units'] = ""hours since 1988-01-01 06:00:00""
da_2.time.attrs['calendar'] = ""proleptic_gregorian""
da_2.time.attrs['standard_name'] = ""time""

ds_2 = xr.Dataset({'V':da_2})
ds_2.attrs['Conventions'] = 'CF'
# saving it with wrong time value
ds_2.to_netcdf('ds_2.nc', encoding=None)


# Reading the 2 files and concatenating them
files = ['/path/to/ds_1.nc', '/path/to/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=(yr), end=str(yr+1), freq='6H', closed='left')
ds_test.time.values=dates[:2] # fixing the time values 
ds_test.time.attrs['units'] = ""Seconds since 1970-01-01 00:00:00""  #required encoding
ds_test.to_netcdf('ds_1_2.nc')  # gives the same error
```
>ValueError: failed to prevent overwriting existing key units in attrs on variable 'time'. This is probably an encoding field used by xarray to describe how a variable is serialized. To proceed, remove this key from the variable's attributes manually.

I've also mentioned your suggestion earlier in the original post. It also gives the same error message
Please find the following reproducible steps incorporating your suggestion.


## Trying time encoding solution 
```
# Reading the files 
files = ['/path/to/ds_1.nc', '/path/to/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=(yr), end=str(yr+1), freq='6H', closed='left')
ds_test.time.values=dates[:2] # fixing the time values 
# encoding try
ds_test.time.encoding['units'] = ""Seconds since 1970-01-01 00:00:00""
ds_test.to_netcdf('ds_1_2.nc')  # gives same error
```
>ValueError: failed to prevent overwriting existing key calendar in attrs on variable 'time'. This is probably an encoding field used by xarray to describe how a variable is serialized. To proceed, remove this key from the variable's attributes manually.


","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,558293655