`sel_time` time selection requires formatting of string for non-normal calendar years

[gmacgilchrist]

Describe the bug
If a year is prior to 1000, such as for the piControl run used for perfect model experiments, sel_time fails when trying to find a time based on the integer of the year. There is a simple fix to implement zfill(4) to format the string from which the time is being selected.

Code Sample

import cftime
import xarray as xr

ds = xr.Dataset(coords={'time':[cftime.DatetimeNoLeap(300,1,1),cftime.DatetimeNoLeap(301,1,1)]})
year_int = ds.time[0].dt.year.values
ds.sel(time=str(year_int))

fails with

ValueError: no ISO-8601 or cftime-string-like match for string: 300

The following change fixes this issue:

import xarray as xr

ds = xr.Dataset(coords={'time':[cftime.DatetimeNoLeap(300,1,1),cftime.DatetimeNoLeap(301,1,1)]})
year_int = ds.time[0].dt.year.values
ds.sel(time=str(year_int).zfill(4))

Happy to implement this PR, but wasn't completely sure if that change is likely to break any other aspects of the code, e.g. for other datasets, and I haven't yet had time to implement the changes to check, so I'm just logging the issue here.

Output of climpred.show_versions()

Details

INSTALLED VERSIONS
------------------
commit: None
python: 3.11.9 | packaged by conda-forge | (main, Apr 19 2024, 18:36:13) [GCC 12.3.0]
python-bits: 64
OS: Linux
OS-release: 4.18.0-553.5.1.el8_10.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

climpred: 2.4.0
xarray: 2023.12.0
pandas: 2.2.2
numpy: 1.26.4
scipy: 1.13.1
cftime: 1.6.3
netcdf4: None
nc_time_axis: 1.4.1
matplotlib: 3.8.2
cf_xarray: 0.9.2
xclim: 0.50.0
dask: 2024.5.0
distributed: 2024.5.0
setuptools: 69.5.1
pip: 24.0
conda: None
IPython: 8.25.0
sphinx: None

[aaronspring]

Open for a PR. Don't see how this should break anything.

But also a bit unsure why a PR would be needed and what alternative workarounds could be, ie. setting new calendar / time coordinate before using climpred. Do you have a sample code example of what's not possible at the moment?