How do the HOS Timers/clocks calculate on-duty and off-duty time?
The timers are separate and are kept separate. The unit itself will always show the driver the timer that is closest to 00:00 in the bottom left hand side of the screen.
The reason the timers are kept on their own is because they are actually different timers, and must be shown as such.
Consider this scenario:
You have 7 hours left on your 70. You just completed a 10 hour break so you have 14 on-duty, 11 driving, and 8 before you need to take a 30 minute break.
You start driving at 05:00 (with no other on-duty status). You stop driving and go immediately into sleeper berth at 10:00. You now have 2 hours left on your 70, and this timer is no longer counting down.
7 hours and 59 minutes later, at 17:59, you start driving. At this point you still have 2 hours on your 70, but only 1 hour and 1 minute left on your 14 hour clock. The timer you need to be concerned with now is your 14, not your 70. The unit will show that you have 1 hour and 1 minute left to drive.
It is for scenarios such as the one above that provides the need for separate timers that work, and are shown individually.