The only sensible alternative to UTC for the system time is TAI, but I don't know of an OS that lets you use TAI for everything. Perhaps some specialist OS used

The only sensible alternative to UTC for the system time is TAI, but I don't know of an OS that lets you use TAI for everything. Perhaps some specialist OS used for satellites? Being able to take the difference of two numerical values to determine the elapsed time, without having to worry about leap seconds, would be so much simpler. You could convert to UTC for display purposes, when possible. It's not always possible for times in the future, but that's a concept people are already used to: you can't reliably convert a future time stamp between local time zones, either.