What happened:

Decoding of times fails for NetCDF4 CfRadial2 style file due to time_reference-string is in another variable.

What you expected to happen:

time_reference should be extracted from the variable and time should be decoded properly.

Minimal Complete Verifiable Example:

```python

create file with such feature

time = np.arange(10) time_attrs = dict(standard_name="time", units="seconds since time_reference", calendar="gregorian") time_ref_attrs = dict(comments="UTC reference date. Format follows ISO 8601 standard.")

ds = xr.Dataset( data_vars=dict( time=(["time"], time, time_attrs),
time_reference=([], '1970-01-01T00:00:00Z', time_ref_attrs), ))

ds.to_netcdf("test.nc")

breaks with the error below

with xr.open_dataset("test.nc") as ds: print(ds)

ad-hoc fix

with xr.open_dataset("test.nc", decode_times=False) as ds: tr = ds.time.attrs["units"].split(" ") nt = ds[tr[-1]].item() ds.time.attrs["units"] = " ".join([*tr[:-1], nt]) ds = xr.decode_cf(ds) print(ds) ```

```python

AttributeError Traceback (most recent call last) ~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/coding/times.py in _decode_cf_datetime_dtype(data, units, calendar, use_cftime) 142 try: --> 143 result = decode_cf_datetime(example_value, units, calendar, use_cftime) 144 except Exception:

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/coding/times.py in decode_cf_datetime(num_dates, units, calendar, use_cftime) 224 try: --> 225 dates = _decode_datetime_with_pandas(flat_num_dates, units, calendar) 226 except (KeyError, OutOfBoundsDatetime, OverflowError):

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/coding/times.py in _decode_datetime_with_pandas(flat_num_dates, units, calendar) 174 --> 175 delta, ref_date = _unpack_netcdf_time_units(units) 176 delta = _netcdf_to_numpy_timeunit(delta)

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/coding/times.py in _unpack_netcdf_time_units(units) 127 delta_units, ref_date = [s.strip() for s in matches.groups()] --> 128 ref_date = _ensure_padded_year(ref_date) 129

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/coding/times.py in _ensure_padded_year(ref_date) 102 matches_start_digits = re.match(r"(\d+)(.*)", ref_date) --> 103 ref_year, everything_else = [s for s in matches_start_digits.groups()] 104 ref_date_padded = "{:04d}{}".format(int(ref_year), everything_else)

AttributeError: 'NoneType' object has no attribute 'groups'

During handling of the above exception, another exception occurred:

ValueError Traceback (most recent call last) <ipython-input-76-da468c6317bb> in <module> 13 ds.to_netcdf("test.nc") 14 ---> 15 with xr.open_dataset("test.nc") as ds: 16 print(ds) 17

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/backends/api.py in open_dataset(filename_or_obj, group, decode_cf, mask_and_scale, decode_times, concat_characters, decode_coords, engine, chunks, lock, cache, drop_variables, backend_kwargs, use_cftime, decode_timedelta) 432 from . import apiv2 433 --> 434 return apiv2.open_dataset(**kwargs) 435 436 if mask_and_scale is None:

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/backends/apiv2.py in open_dataset(filename_or_obj, engine, chunks, cache, decode_cf, mask_and_scale, decode_times, decode_timedelta, use_cftime, concat_characters, decode_coords, drop_variables, backend_kwargs, **kwargs) 266 267 overwrite_encoded_chunks = kwargs.pop("overwrite_encoded_chunks", None) --> 268 backend_ds = backend.open_dataset( 269 filename_or_obj, 270 drop_variables=drop_variables,

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/backends/netCDF4_.py in open_dataset(self, filename_or_obj, mask_and_scale, decode_times, concat_characters, decode_coords, drop_variables, use_cftime, decode_timedelta, group, mode, format, clobber, diskless, persist, lock, autoclose) 557 store_entrypoint = StoreBackendEntrypoint() 558 with close_on_error(store): --> 559 ds = store_entrypoint.open_dataset( 560 store, 561 mask_and_scale=mask_and_scale,

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/backends/store.py in open_dataset(self, store, mask_and_scale, decode_times, concat_characters, decode_coords, drop_variables, use_cftime, decode_timedelta) 23 encoding = store.get_encoding() 24 ---> 25 vars, attrs, coord_names = conventions.decode_cf_variables( 26 vars, 27 attrs,

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/conventions.py in decode_cf_variables(variables, attributes, concat_characters, mask_and_scale, decode_times, decode_coords, drop_variables, use_cftime, decode_timedelta) 510 and stackable(v.dims[-1]) 511 ) --> 512 new_vars[k] = decode_cf_variable( 513 k, 514 v,

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/conventions.py in decode_cf_variable(name, var, concat_characters, mask_and_scale, decode_times, decode_endianness, stack_char_dim, use_cftime, decode_timedelta) 358 var = times.CFTimedeltaCoder().decode(var, name=name) 359 if decode_times: --> 360 var = times.CFDatetimeCoder(use_cftime=use_cftime).decode(var, name=name) 361 362 dimensions, data, attributes, encoding = variables.unpack_for_decoding(var)

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/coding/times.py in decode(self, variable, name) 515 units = pop_to(attrs, encoding, "units") 516 calendar = pop_to(attrs, encoding, "calendar") --> 517 dtype = _decode_cf_datetime_dtype(data, units, calendar, self.use_cftime) 518 transform = partial( 519 decode_cf_datetime,

~/miniconda3/envs/h5py3/lib/python3.9/site-packages/xarray/coding/times.py in _decode_cf_datetime_dtype(data, units, calendar, use_cftime) 151 "if it is not installed." 152 ) --> 153 raise ValueError(msg) 154 else: 155 dtype = getattr(result, "dtype", np.dtype("object"))

ValueError: unable to decode time units 'seconds since time_reference' with "calendar 'gregorian'". Try opening your dataset with decode_times=False or installing cftime if it is not installed. ```

Anything else we need to know?:

I've searched the issues to no avail, also internet search wasn't successful so far. Any pointers welcome.

Environment:

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

INSTALLED VERSIONS ------------------ commit: None python: 3.9.0 | packaged by conda-forge | (default, Oct 14 2020, 22:59:50) [GCC 7.5.0] python-bits: 64 OS: Linux OS-release: 5.8.0-43-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: de_DE.UTF-8 LOCALE: de_DE.UTF-8 libhdf5: 1.10.6 libnetcdf: 4.7.4 xarray: 0.16.3.dev124+g0a309e07.d20210218 pandas: 1.2.1 numpy: 1.20.0 scipy: 1.6.0 netCDF4: 1.5.4 pydap: None h5netcdf: 0.10.0 h5py: 3.1.0 Nio: None zarr: None cftime: 1.4.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: 1.3.2 dask: 2.30.0 distributed: 2.30.1 matplotlib: 3.3.2 cartopy: 0.18.0 seaborn: None numbagg: None pint: None setuptools: 49.6.0.post20201009 pip: 20.2.4 conda: None pytest: 6.1.2 IPython: 7.19.0 sphinx: None