hmm, I still like the idea of doing
~~~ scala
val timeOfDay = millis / 86400000L
val hours = timeOfDay / 3600000L
val minutes = timeOfDay / 60000L % 60
val seconds = timeOfDay / 1000L % 60
val ms = timeOfDay % 1000
val timestamp = f"$hours%02d:$minutes%02d:$seconds%02d.$ms%03dUTC"
~~~
because it is more useful as a printed value than the millis themselves, and it still is reasonably cheap; maybe put both in the MDC?