xarray v 2023.9.0: ```ValueError: unable to infer dtype on variable 'time'; xarray cannot serialize arbitrary Python objects```

What happened?

I tried to save an xarray dataset with datetimes as data for its time dimension to a nc file with to_netcdf and got the error ValueError: unable to infer dtype on variable 'time'; xarray cannot serialize arbitrary Python objects.

What did you expect to happen?

I expected xarray to automatically detect these were datetimes, and convert them to whatever format xarray likes to work with internally to dump it into a CF compatible file, following what is described at https://github.com/pydata/xarray/issues/2512 .

Minimal Complete Verifiable Example

import xarray as xr
import datetime

times = [datetime.datetime(2024, 1, 1, 1, 1, 1, tzinfo=datetime.timezone.utc), datetime.datetime(2024, 1, 1, 1, 1, 2, tzinfo=datetime.timezone.utc)]

data = [1, 2]

xr_result = xr.Dataset(
    {
        'time':
        xr.DataArray(dims=["time"],
                     data=times,
                     attrs={
                         "standard_name": "time",
                     }),
        #
        'data':
        xr.DataArray(dims=["time"],
                     data=data,
                     attrs={
                         "_FillValue": "NaN",
                         "standard_name": "some_data",
                     }),
    }
)

xr_result.to_netcdf("test.nc")

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?

The example is available as a notebook viewable at:

https://github.com/jerabaul29/public_bug_reports/blob/main/xarray/2024_01_24/xarray_and_datetimes.ipynb

Environment

Bắt đầu với ví dụ tối giản được cung cấp và lần theo đường đi của Dataset.to_netcdf đối với biến time có giá trị datetime. Xác nhận hành vi bằng một regression test tập trung và coi issue là hoàn tất khi ví dụ được tuần tự hóa thành công thành một tệp netCDF tương thích với CF mà không có lỗi dtype.