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
868907284,MDU6SXNzdWU4Njg5MDcyODQ=,5223,'NaT' as fill value and netcdf export,6503378,closed,0,,,19,2021-04-27T14:29:16Z,2021-05-04T14:02:32Z,2021-05-04T14:02:32Z,NONE,,,,"I do not understand why this (xarray 0.17.0)

```
da = xr.DataArray([np.datetime64('NAT')])
da.to_netcdf('da')
```
is OK and the following gives an error

```
da = xr.DataArray([np.datetime64('NAT')], attrs={'_FillValue': np.datetime64('NAT')})
da.to_netcdf('da')
```

`TypeError: Invalid value for attr '_FillValue': numpy.datetime64('NaT') must be a number, a string, an ndarray or a list/tuple of numbers/strings for serialization to netCDF files`

What is a ""proper"" fill value for datedime64? Thank you, Jan

_Originally posted by @jvail in https://github.com/pydata/xarray/discussions/5200_

I could not get an answer in the Q&A section. Essentially: I find it puzzling that something acceptable as a value is unacceptable as fill value.","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/5223/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue