[488643689]

That's a smart thought, I haven't had!

Could be testable by turning the stopwatch on and then change the time. I have no idea how these clocks work; if they have a Unix time style counter as internal reference, then this test won't work. Would need a 36 bit memory to run for 20 years continuous counting; 30 billion times bit flipping on the lowest bit. Probably done differently.