The UTC timestamp feature is newer than the joulescope API. While the timestamp support exists in the pyjoulescope_driver package, it does not exist in the joulescope package. Can you record using the pyjoulescope_driver package:
python -m pyjoulescope_driver record --signals i,v,p out.jls
If you want to run this from within a script, you can use the Record class.
For what it’s worth, this is pyjoulescope issue #37. While writing UTC time to the JLS file is easy using the Writer.utc method, UTC time simply does not exist for the pyjoulescope StreamBuffer.