Well, my discoveries so far:
Hold the clock (RH) button down first (and hold) then press and hold the trip button (do it the other way and you will reset your reading) together.
If you have ignition on I you get a series of numbers (LH display) with 1.1, 1.2 etc. reading on the RH side as you continue to press the button.
Ignition on II reads "test" on the LH display and then as the button is pressed, numbers 1 through 21 (and then 0) are displayed.
What's it all mean ?
Oh, yeah, if you hold the trip button while turning on the ignition, the self test goes through the motions.