xr.cftime_range fails when using proleptic_gregorian calendar in cftime.datetime object

[awikner]

What happened?

I want to use the xarray.cftime_range function to get a set of datetimes between two cftime.datetime objects that use the proleptic_gregorian calendar. However, when I use this function with this calendar, I receive the following error:

File "/Users/Alexander/miniconda3/envs/xrtest/lib/python3.11/site-packages/xarray/core/resample_cftime.py", line 491, in exact_cftime_datetime_difference
    seconds = b.replace(microsecond=0) - a.replace(microsecond=0)
              ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
  File "src/cftime/_cftime.pyx", line 1574, in cftime._cftime.datetime.__sub__
TypeError: cannot compute the time difference between dates with different calendars

What did you expect to happen?

I expect this function to return a list of dates between the two dates I selected, with each date spaced by one day. In the minimal example, I provide the expected output obtained when I instead use the cftime.DatetimeProlepticGregorian object.

Minimal Complete Verifiable Example

#xarray==2024.3.0
#cftime==1.6.3
import xarray as xr
import cftime

start_date = cftime.DatetimeProlepticGregorian(1,1,1)
end_date = cftime.DatetimeProlepticGregorian(2,1,1)

daterange = xr.cftime_range(start_date, end_date, calendar='proleptic_gregorian')
print(daterange)

start_date = cftime.datetime(1,1,1,calendar = 'proleptic_gregorian')
end_date = cftime.datetime(2,1,1,calendar = 'proleptic_gregorian')

daterange = xr.cftime_range(start_date, end_date, calendar='proleptic_gregorian')
print(daterange)

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.11.8 | packaged by conda-forge | (main, Feb 16 2024, 20:49:36) [Clang 16.0.6 ] python-bits: 64 OS: Darwin OS-release: 23.4.0 machine: arm64 processor: arm byteorder: little LC_ALL: None LANG: None LOCALE: ('en_US', 'UTF-8') libhdf5: None libnetcdf: None xarray: 2024.3.0 pandas: 2.2.2 numpy: 1.26.4 scipy: None netCDF4: None pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.6.3 nc_time_axis: None iris: None bottleneck: None dask: None distributed: None matplotlib: None cartopy: None seaborn: None numbagg: None fsspec: None cupy: None pint: None sparse: None flox: None numpy_groupies: None setuptools: 69.2.0 pip: 24.0 conda: None pytest: None mypy: None IPython: None sphinx: None

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[spencerkclark]

Indeed for the time being xarray only supports the calendar-specific cftime.datetime subclasses like cftime.DatetimeProlepticGregorian. I started a PR to enable support for calendar-aware cftime.datetime instances in #8942. In principle it should not be too hard, though it will take some time to make sure everything is tested thoroughly enough.