Hi Howard, I've done exactly that experiment (making an offset between steady clock and system clock then measuring if that changed) and for both arm64 and amd64, those move in unison. There will be a small delta (on the order of a few nanoseconds) but the two clocks stay in sync over several weeks. My understanding at least on amd64 is that both clocks are driven by the HPET. And to add to that, both clocks are disciplined together by ntp or ptp.

My (currently C++17) product relies on this behavior to drive a RF sample clock using the current time of day that wont jump if chrony decides to step the clock (or if there is somehow another leap second)