```python

Imports

import numpy as np import xarray as xr import pandas as pd from glob import glob

files to be concatenated

files = sorted(glob(path + str(1988) + '/V250*'))

corrected dates

dates = pd.date_range(start=str(yr), end=str(yr+1), freq='6H', closed='left')

ds_test = xr.open_mfdataset(files[:10], combine='nested', concat_dim='time', decode_cf=False)

correcting time

ds_test.time.values=dates[:10]

fixing encoding

ds_test.time.attrs['units'] = "Seconds since 1970-01-01 00:00:00"

preview of the time variable

print(ds_test.time)

<xarray.DataArray 'time' (time: 10)> array(['1988-01-01T00:00:00.000000000', '1988-01-01T06:00:00.000000000', '1988-01-01T12:00:00.000000000', '1988-01-01T18:00:00.000000000', '1988-01-02T00:00:00.000000000', '1988-01-02T06:00:00.000000000', '1988-01-02T12:00:00.000000000', '1988-01-02T18:00:00.000000000', '1988-01-03T00:00:00.000000000', '1988-01-03T06:00:00.000000000'], dtype='datetime64[ns]') Coordinates: * time (time) datetime64[ns] 1988-01-01 ... 1988-01-03T06:00:00 Attributes: calendar: proleptic_gregorian standard_name: time units: Seconds since 1970-01-01 00:00:00

ds_test.to_netcdf(path+'test.nc')

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.

```

Expected Output

Correctly encode time such that it saves the file by correctly converting value of time according to the reference units. I have the flexibility of dropping CF-conventions as long as time values are correct but it would also be nice to have a solution which keeps the CF-conventions intact.

Problem Description

I'm trying to concatenate netcdf files which have CF conventions mentioned in their global attributes. These files have an incorrect time dimension which I try to fix with the code above. It seems that some existing encoding is preventing from writing the files back. But when I print the encoding, it doesn't show any such clashing units. I'm not sure if this is a bug or a wrong usage issue. Thus, any usage help on how to correctly encode time such that it saves the time values by correctly converting according to the reference units is much appreciated.

```python

More diagnostics on the encoding

print(ds_test.encoding)

{'unlimited_dims': {'time'}, 'source': '/file/to/path/V250_19880101_00'}

checking any existing time

print(ds_test.time.encoding)

{}

another try on setting time encoding

ds_test.time.encoding['units'] = "Seconds since 1970-01-01 00:00:00"

writing the file gives the same ValueError as above

ds_test.to_netcdf(path+'test.nc')

ncdump output of one of the files

netcdf V250_19880101_06 { dimensions: lon = 720 ; lat = 361 ; lev = 1 ; time = UNLIMITED ; // (1 currently) variables: float lon(lon) ; lon:long_name = "longitude" ; lon:units = "degrees_east" ; lon:standard_name = "longitude" ; lon:axis = "X" ; float lat(lat) ; lat:long_name = "latitude" ; lat:units = "degrees_north" ; lat:standard_name = "latitude" ; lat:axis = "Y" ; float lev(lev) ; lev:long_name = "hybrid level at layer midpoints" ; lev:units = "level" ; lev:standard_name = "hybrid_sigma_pressure" ; lev:positive = "down" ; lev:formula = "hyam hybm (mlev=hyam+hybm*aps)" ; lev:formula_terms = "ap: hyam b: hybm ps: aps" ; float time(time) ; time:units = "hours since 1988-01-01 06:00:00" ; time:calendar = "proleptic_gregorian" ; time:standard_name = "time" ; float V(time, lev, lat, lon) ; V:long_name = "unknown (please add with NCO)" ; V:units = "unknown (please add with NCO)" ; V:_FillValue = -999.99f ;

// global attributes: :Conventions = "CF" ; :constants_file_name = "P19880101_06" ; :institution = "IACETH" ; :lonmin = -180.f ; :lonmax = 179.5f ; :latmin = -90.f ; :latmax = 90.f ; :levmin = 250.f ; :levmax = 250.f ; :history = "Fri Sep 6 15:59:17 2019: ncatted -a units,time,o,c,hours since 1988-01-01 06:00:00 -a standard_name,time,o,c,time V250_19880101_06" ; :NCO = "4.7.2" ; data:

time = 6 ; }

```

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.4 (default, Aug 13 2019, 20:35:49) [GCC 7.3.0] python-bits: 64 OS: Linux OS-release: 5.0.0-23-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.1 xarray: 0.13.0 pandas: 0.25.3 numpy: 1.18.1 scipy: 1.3.2 netCDF4: 1.4.2 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.0.4.2 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.9.2 distributed: 2.9.3 matplotlib: 3.1.0 cartopy: 0.17.0 seaborn: 0.9.0 numbagg: None setuptools: 44.0.0.post20200106 pip: 19.3.1 conda: None pytest: None IPython: 7.11.1 sphinx: None