Value based indexing using .sel is broken on datetime coords

[philippjfr]

What happened:

Value based indexing using .sel on a datetime coordinate is broken.

Minimal Complete Verifiable Example:

import numpy as np
import pandas
import xarray as xr

a = xr.DataArray(np.arange(27).reshape(3,3,3), 
                 dims=["x", "y", "time"], 
                 coords=dict(x=[1,2,3], y=[1,2,3], time=pd.date_range("2020-01-01", freq="1d", periods=3)))

a.sel(time=a.time.values[0])

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-10-b52cbbe5c7df> in <module>
----> 1 a.sel(time=a.time.values[0])

~/miniconda3/envs/gv_dev/lib/python3.7/site-packages/xarray/core/dataarray.py in sel(self, indexers, method, tolerance, drop, **indexers_kwargs)
   1059             method=method,
   1060             tolerance=tolerance,
-> 1061             **indexers_kwargs,
   1062         )
   1063         return self._from_temp_dataset(ds)

~/miniconda3/envs/gv_dev/lib/python3.7/site-packages/xarray/core/dataset.py in sel(self, indexers, method, tolerance, drop, **indexers_kwargs)
   2064         indexers = either_dict_or_kwargs(indexers, indexers_kwargs, "sel")
   2065         pos_indexers, new_indexes = remap_label_indexers(
-> 2066             self, indexers=indexers, method=method, tolerance=tolerance
   2067         )
   2068         result = self.isel(indexers=pos_indexers, drop=drop)

~/miniconda3/envs/gv_dev/lib/python3.7/site-packages/xarray/core/coordinates.py in remap_label_indexers(obj, indexers, method, tolerance, **indexers_kwargs)
    395 
    396     pos_indexers, new_indexes = indexing.remap_label_indexers(
--> 397         obj, v_indexers, method=method, tolerance=tolerance
    398     )
    399     # attach indexer's coordinate to pos_indexers

~/miniconda3/envs/gv_dev/lib/python3.7/site-packages/xarray/core/indexing.py in remap_label_indexers(data_obj, indexers, method, tolerance)
    268             coords_dtype = data_obj.coords[dim].dtype
    269             label = maybe_cast_to_coords_dtype(label, coords_dtype)
--> 270             idxr, new_idx = convert_label_indexer(index, label, dim, method, tolerance)
    271             pos_indexers[dim] = idxr
    272             if new_idx is not None:

~/miniconda3/envs/gv_dev/lib/python3.7/site-packages/xarray/core/indexing.py in convert_label_indexer(index, label, index_name, method, tolerance)
    188             else:
    189                 indexer = index.get_loc(
--> 190                     label.item(), method=method, tolerance=tolerance
    191                 )
    192         elif label.dtype.kind == "b":

~/miniconda3/envs/gv_dev/lib/python3.7/site-packages/pandas/core/indexes/datetimes.py in get_loc(self, key, method, tolerance)
    620         else:
    621             # unrecognized type
--> 622             raise KeyError(key)
    623 
    624         try:

KeyError: 1577836800000000000

Anything else we need to know?:

Environment:

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.7.5 (default, Oct 25 2019, 10:52:18)
[Clang 4.0.1 (tags/RELEASE_401/final)]
python-bits: 64
OS: Darwin
OS-release: 18.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.2

xarray: 0.16.0
pandas: 1.1.1
numpy: 1.17.3
scipy: 1.4.1
netCDF4: 1.5.1.2
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: 2.3.2
cftime: 1.0.4.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.1.4
cfgrib: None
iris: None
bottleneck: None
dask: 2.9.0
distributed: 2.9.0
matplotlib: 3.1.2
cartopy: 0.17.0
seaborn: 0.10.0
numbagg: None
pint: None
setuptools: 46.1.3.post20200330
pip: 19.3.1
conda: 4.8.3
pytest: 5.3.2
IPython: 7.10.2
sphinx: 2.3.1

[philippjfr]

Apologies, duplicate of #4283. Closing.