issues: 702373263

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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
702373263 MDU6SXNzdWU3MDIzNzMyNjM= 4427 assign_coords with datetime64[us] changes dtype to datetime64[ns] 22488770 closed 0     3 2020-09-16T01:14:11Z 2020-09-30T00:49:35Z 2020-09-30T00:49:35Z CONTRIBUTOR      

What happened: When using xr.DataArray.assign_coords() to assign a new coordinate to the time dimension that is an array with dtype datetime64[us], after assigning, the dtype is datetime64[ns], resulting in the wrong dates, since the dates I am using are outside the valid range for the [ns] units.

What you expected to happen: Preserve dtype of array when assigning as a coordinate.

Minimal Complete Verifiable Example:

```python import numpy as np import xarray as xr import cftime

tmp = np.random.random(12)

da = xr.DataArray(tmp, dims='time')

times=list()

for mth in np.arange(1, 13): times.append(cftime.DatetimeNoLeap(1250, mth, 1))

times64 = np.array([np.datetime64(t, 'us') for t in times])

da = da.assign_coords({'time': times64}) which gives for the original array:python In [49]: times64 Out[49]: array(['1250-01-01T00:00:00.000000', '1250-02-01T00:00:00.000000', '1250-03-01T00:00:00.000000', '1250-04-01T00:00:00.000000', '1250-05-01T00:00:00.000000', '1250-06-01T00:00:00.000000', '1250-07-01T00:00:00.000000', '1250-08-01T00:00:00.000000', '1250-09-01T00:00:00.000000', '1250-10-01T00:00:00.000000', '1250-11-01T00:00:00.000000', '1250-12-01T00:00:00.000000'], dtype='datetime64[us]') ```

and for the array after assigning: python In [51]: da.time Out[51]: <xarray.DataArray 'time' (time: 12)> array(['1834-07-22T23:34:33.709551616', '1834-08-22T23:34:33.709551616', '1834-09-19T23:34:33.709551616', '1834-10-20T23:34:33.709551616', '1834-11-19T23:34:33.709551616', '1834-12-20T23:34:33.709551616', '1835-01-19T23:34:33.709551616', '1835-02-19T23:34:33.709551616', '1835-03-22T23:34:33.709551616', '1835-04-21T23:34:33.709551616', '1835-05-22T23:34:33.709551616', '1835-06-21T23:34:33.709551616'], dtype='datetime64[ns]') Coordinates: * time (time) datetime64[ns] 1834-07-22T23:34:33.709551616 ... 1835-06-...

Anything else we need to know?:

Environment:

Output of <tt>xr.show_versions()</tt> INSTALLED VERSIONS ------------------ commit: None python: 3.7.8 | packaged by conda-forge | (default, Jul 31 2020, 02:37:09) [Clang 10.0.1 ] python-bits: 64 OS: Darwin OS-release: 18.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: en_US.UTF-8 LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.7.3 xarray: 0.16.0 pandas: 1.1.0 numpy: 1.19.1 scipy: 1.4.1 netCDF4: 1.5.3 pydap: installed h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.0.4.2 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.21.0 distributed: 2.22.0 matplotlib: 3.1.2 cartopy: 0.17.0 seaborn: None numbagg: None pint: None setuptools: 49.3.1.post20200810 pip: 20.2.2 conda: None pytest: None IPython: 7.17.0 sphinx: 3.2.0 
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