How to get fractions of seconds?

[Stefan A.]

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