I've looked at the module "times", but do not see how to emulate Java's System.currentTimeMillis() with it, which is basically what I'd need.
I've been working on various improvements for the times module (https://github.com/nim-lang/Nim/pull/6552 ). Sub second resolution is not part of that PR, but my plan is to implement that next. The current PR is more or less ready for merge, so unless someone beats me to it, I'll likely implement sub second resolution within the next month or so (and that implementation will satisfy those requirements).
For now, you'll have to wrap the necessary C functions yourself.
include "$lib/system/timers.nim"
That is what the realtime GC uses and should become a stdlib module.