Simple example:

```python import numpy as np from datetime import datetime, timedelta import xarray

"time" coordinate

dt = datetime(1999, 1, 1) dts = np.array([dt + timedelta(days=x) for x in range(10)]) coords = {'time': dts}

simple float data

data = np.arange(10) vrbls = {'foo': (('time',), data)}

create the Dataset

ds = xarray.Dataset(vrbls, coords)

encode the time coordinate

units = 'days since 1900-01-01' ds.time.encoding['units'] = units

write to netcdf

ds.to_netcdf('test.nc')

```

Problem description

When I run the above, I get the following error when executing the last line: ValueError: unsupported dtype for netCDF4 variable: datetime64[ns]

The documentation indicates that datetime and datetime64 objects are both supported by xarray and should write to netcdf just fine when supplied "units" for encoding (this code fails with or without the encoding lines). Any Idea when is going wrong here?

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.2.final.0 python-bits: 64 OS: Linux OS-release: 4.9.0-8-amd64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 xarray: 0.10.9 pandas: 0.20.3 numpy: 1.13.1 scipy: 0.19.1 netCDF4: 1.4.2 h5netcdf: 0.5.0 h5py: 2.8.0 Nio: None zarr: None cftime: 1.0.1 PseudonetCDF: None rasterio: None iris: None bottleneck: 1.2.1 cyordereddict: None dask: 0.16.0 distributed: 1.20.1 matplotlib: 2.1.0 cartopy: None seaborn: 0.8.0 setuptools: 27.2.0 pip: 9.0.1 conda: 4.5.11 pytest: 3.1.3 IPython: 6.1.0 sphinx: 1.6.2