Are not all aviation time-dependent systems for ATC etc. systems synchronised to a single unit time source, e.g. the Rugby Atomic Clock or similar?
Most timing signals for this sort of equipment are driven from GPS. In fact by far the biggest use of GPS is not navigation, but rather an accurate time signal.
Any source for the claim that most aviation timing signals are driven from GPS? I'm not so sure GPS time is used much on the aircraft side of things.
I'm a pilot, not a mechanic or avionics technician, but various clocks on the airliners I've flown are usually manually set and can often disagree, especially when the aircraft battery has been recently disconnected for maintenance.
ACARS has a clock display but I'm under the impression that it is updated by a VHF, HFDL or SATCOM signal, I don't think it even sees GPS information.
The FMS's get GPS position information but I've seen the clocks disagree from the actual time when the captain's instrument panel clock is not set correctly. It's a real mess with CPDLC, this may be a legacy issue corrected on newer aircraft designs. However the B-777 was designed and spec'ed years ago.
We still have the old HF time hack in the overwater preflight procedures, probably a carryover from a Pan Am manual many decades ago.