You're right, JJay. I've googled on the topic a lot. The total time a machine has been on was supported by Apple computers in the nineties only. Today System 10 doesn't offer it any longer. As an ex-Apple developer, I remember pretty well seeing that piece of information in System 9. Obviously, at that time, no one could change OS...
Then I wrongly assumed the information was accessible on all computers but it's not. Not even in the BIOS. Yet I doubt computer manufacturers don't keep the time-on somewhere because it's a crucial piece of information with defective computers.
In fact, the only time-on users can get is the one since the last system install. What Everest provides on PC. The following thread may show how to calculate it in a Linux environment.
https://discussions.apple.com/thread/49 ... 0&tstart=0