Otto,
If you have a process that spans days do you "really" need millisecond resolution? You can have unrelated processes that kick off on the machine during the runtime of the process that can affect the runtime by seconds. 1 second is 0.00116% or a day, if you are trying to measure calendar-worthy elapsed time to better accuracy you are wasting time and effort IMO.
>that I was dissatisfied with both SECONDS() and DATETIME() for general elapsed-time measurement - the former because it resets at midnight and the latter because of its coarse resolution of 1 second. One can, of course, combine the two to get millisecond resolution over calender-worthy time spans but it is a bit of a hassle.