id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
1182697604,I_kwDOAMm_X85GfoiE,6416,xr.concat removes datetime information,14371165,closed,0,,,2,2022-03-27T23:19:30Z,2022-03-28T16:05:01Z,2022-03-28T16:05:01Z,MEMBER,,,,"### What happened?

xr.concat removes datetime information and can't concatenate the arrays because they don't have compatible types anymore.

### What did you expect to happen?

Succesful concatenation with the same type.

### Minimal Complete Verifiable Example

```Python
import numpy as np
import xarray as xr
from datetime import datetime

month = np.arange(1, 13, 1)
data = np.sin(2 * np.pi * month / 12.0)

darray = xr.DataArray(data, dims=[""time""])
darray.coords[""time""] = np.array([datetime(2017, m, 1) for m in month])

darray_nan = np.nan * darray.isel(**{""time"": -1})
darray = xr.concat([darray, darray_nan], dim=""time"")
```


### Relevant log output

```Python
Traceback (most recent call last):

  File ""<ipython-input-15-31040255a336>"", line 2, in <module>
    darray = xr.concat([darray, darray_nan], dim=""time"")

  File ""c:\users\j.w\documents\github\xarray\xarray\core\concat.py"", line 244, in concat
    return f(

  File ""c:\users\j.w\documents\github\xarray\xarray\core\concat.py"", line 642, in _dataarray_concat
    ds = _dataset_concat(

  File ""c:\users\j.w\documents\github\xarray\xarray\core\concat.py"", line 555, in _dataset_concat
    combined_idx = indexes[0].concat(indexes, dim, positions)

  File ""c:\users\j.w\documents\github\xarray\xarray\core\indexes.py"", line 318, in concat
    coord_dtype = np.result_type(*[idx.coord_dtype for idx in indexes])

  File ""<__array_function__ internals>"", line 5, in result_type

TypeError: The DType <class 'numpy.dtype[datetime64]'> could not be promoted by <class 'numpy.dtype[int64]'>. This means that no common DType exists for the given inputs. For example they cannot be stored in a single array unless the dtype is `object`. The full list of DTypes is: (<class 'numpy.dtype[datetime64]'>, <class 'numpy.dtype[int64]'>)
```


### Anything else we need to know?

Similar to #6384.

Happens around here:

https://github.com/pydata/xarray/blob/728b648d5c7c3e22fe3704ba163012840408bf66/xarray/core/concat.py#L535

### Environment

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.9.6 | packaged by conda-forge | (default, Jul 11 2021, 03:37:25) [MSC v.1916 64 bit (AMD64)]
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: ('Swedish_Sweden', '1252')
libhdf5: 1.10.6
libnetcdf: 4.7.4

xarray: 0.16.3.dev99+gc19467fb
pandas: 1.3.1
numpy: 1.21.5
scipy: 1.7.1
netCDF4: 1.5.6
pydap: installed
h5netcdf: 0.11.0
h5py: 2.10.0
Nio: None
zarr: 2.8.3
cftime: 1.5.0
nc_time_axis: 1.3.1
PseudoNetCDF: installed
rasterio: 1.2.6
cfgrib: None
iris: 3.0.4
bottleneck: 1.3.2
dask: 2021.10.0
distributed: 2021.10.0
matplotlib: 3.4.3
cartopy: 0.19.0.post1
seaborn: 0.11.1
numbagg: 0.2.1
fsspec: 2021.11.1
cupy: None
pint: 0.17
sparse: 0.12.0
setuptools: 49.6.0.post20210108
pip: 21.2.4
conda: None
pytest: 6.2.4
IPython: 7.31.0
sphinx: 4.3.2

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