How to get fractions of seconds?

Good morning, for logging and monitoring purposes, I need to add the time spend for a particular Flow to a Logfile. Best would be to have something like '%l', but in a resolution of milliseconds. If I would have the runtime variable of (e.g.) %t for "timestamp.milliseconds" I could try something like update control { FlowStartTime = %t} ...do the Flow... update control {UsedTime = `%{expr: %t - control:FlowStartTime}` } Is there something like this or a similar solution? Thank you. Regards Stefan