What happened?

Using xarray.open_dataset. Trying to read a Netcdf dataset with generic time coordinates, not CF, with time:units = "seconds". Trying to apply the conversion "time_unit='us'" as directed by a warning message.

• Got an invalid "SerializationWarning" message.
• Argument "time_unit='us'" seems to be ignored. Time is converted to nanosecs rather than microsecs.

What did you expect to happen?

• After input, time coordinates should be microsecs rather than nanosecs.
• Clean execution with no warning message.

Minimal Complete Verifiable Example

#!/usr/bin/env python3
import xarray as xr
coder = xr.coders.CFDatetimeCoder (use_cftime=False, time_unit='us')
IN = xr.open_dataset ('xtime.nc', decode_timedelta=coder)
print (IN)

# See below for source code to generate the simple "xtime.nc" test file.

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

(test_env) > ./demo.time.2.py
/home/xxxxxx/demo.time.2.py:4: SerializationWarning: Can't decode floating point timedelta to 's' without precision loss, decoding to 'us' instead. To silence this warning use time_unit='us' in call to decoding function.
  IN = xr.open_dataset ('xtime.nc', decode_timedelta=coder)
<xarray.Dataset> Size: 40B
Dimensions:  (time: 5)
Coordinates:
  * time     (time) timedelta64[ns] 40B 00:00:48.561388 ... 00:01:47.403320
Data variables:
    *empty*

Anything else we need to know?

Source code for xtime.nc:

netcdf xtime {
dimensions:
	time = 5 ;
variables:
	float time(time) ;
		time:units = "seconds" ;
data:
 time = 48.56139, 63.27169, 77.98219, 92.6929, 107.4033 ;
}

Generate the actual test file using this command:

ncgen -o xtime.nc xtime.txt

Environment