Bugfix read netcdf not working on single time step zonal mean data

[CPKalb]

Describe the Problem

METdataio read_netcd work for zonal mean data when an array of times is input. However, with a single time step, the script fails with the following error:
Traceback (most recent call last):
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/coding/times.py", line 285, in decode_cf_datetime
dates = _decode_datetime_with_pandas(flat_num_dates, units, calendar)
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/coding/times.py", line 218, in _decode_datetime_with_pandas
raise OutOfBoundsDatetime(
pandas._libs.tslibs.np_datetime.OutOfBoundsDatetime: Cannot decode times from a non-standard calendar, 'julian', using pandas.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/coding/times.py", line 184, in _decode_cf_datetime_dtype
result = decode_cf_datetime(example_value, units, calendar, use_cftime)
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/coding/times.py", line 287, in decode_cf_datetime
dates = _decode_datetime_with_cftime(
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/coding/times.py", line 208, in _decode_datetime_with_cftime
cftime.num2date(num_dates, units, calendar, only_use_cftime_datetimes=True)
File "src/cftime/_cftime.pyx", line 617, in cftime._cftime.num2date
File "src/cftime/_cftime.pyx", line 414, in cftime._cftime.cast_to_int
OverflowError: time values outside range of 64 bit signed integers

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/conventions.py", line 551, in decode_cf_variables
new_vars[k] = decode_cf_variable(
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/conventions.py", line 397, in decode_cf_variable
var = times.CFDatetimeCoder(use_cftime=use_cftime).decode(var, name=name)
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/coding/times.py", line 716, in decode
dtype = _decode_cf_datetime_dtype(data, units, calendar, self.use_cftime)
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/coding/times.py", line 194, in _decode_cf_datetime_dtype
raise ValueError(msg)
ValueError: unable to decode time units 'days since 2018-02-12' with "calendar 'julian'". Try opening your dataset with decode_times=False or installing cftime if it is not installed.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/glade/u/home/kalb/stratosphere/METplus/parm/use_cases/model_applications/s2s_stratosphere/UserScript_fcstGFS_obsERA_StratospherePolar/polar_t_u_driver.py", line 112, in
main()
File "/glade/u/home/kalb/stratosphere/METplus/parm/use_cases/model_applications/s2s_stratosphere/UserScript_fcstGFS_obsERA_StratospherePolar/polar_t_u_driver.py", line 47, in main
dsO = file_reader.read_into_xarray(obs_infiles)[0]
File "/glade/u/home/kalb/stratosphere/METdataio/METreadnc/util/read_netcdf.py", line 70, in read_into_xarray
file_data = xr.open_dataset(file)
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/backends/api.py", line 541, in open_dataset
backend_ds = backend.open_dataset(
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/backends/netCDF4_.py", line 592, in open_dataset
ds = store_entrypoint.open_dataset(
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/backends/store.py", line 35, in open_dataset
vars, attrs, coord_names = conventions.decode_cf_variables(
File "/glade/work/dtcrt/METplus/derecho/miniconda/miniconda3/envs/metplus_v5.1_py3.10/lib/python3.10/site-packages/xarray/conventions.py", line 562, in decode_cf_variables
raise type(e)(f"Failed to decode variable {k!r}: {e}")
ValueError: Failed to decode variable 'time': unable to decode time units 'days since 2018-02-12' with "calendar 'julian'". Try opening your dataset with decode_times=False or installing cftime if it is not installed.

Expected Behavior

It should successfully read data with a single time step or an array of times

Environment

Describe your runtime environment:
*1. derecho
*2. linux
3. Software version number(s)

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