Maybe the clock can be made to drift? You can make the clock a random walk with the restriction that it never decreases, and doesn't deviate more than 20us? Tha

Maybe the clock can be made to drift? You can make the clock a random walk with the restriction that it never decreases, and doesn't deviate more than 20us? That is, each tick of the clock actually just adds a non-negative interval to the clock. Since the deviation from real time at tick depends on the deviation at tick-1 it would require more time to get good accuracy by averaging measurements.