The purpose of `$TV(un)` is to get the microseconds when `$TV(sn)` is computed, because the time at that moment is `$TV(sn).$TV(un)`. The `$TV(un)` is not the time at that moment in micro-seconds, but the micro-seconds part of the time.
If you need time in microseconds, you need to add a new variable. `$TV(un)` has to stay like it is. I will try to make it clear in the docs.