one of the Voyager's became 12 years ahead of schedule

I don't believe this. I even think Colin2B's 2% error margin is

*far* too pessimistic.

In the Solar System, outside Earth's orbit, Newton's law of universal gravitation is extremely accurate. If you are going near the orbit of Mercury, it would be advisable to use Einstein's tiny tweak due to time dilation deep in the Sun's gravitational well (but this is so easy to apply that I am sure NASA used it even on the Voyager journey).

The mass of Jupiter and Saturn (in fact any astronomical body with a natural satellite) was known quite accurately well before Voyager arrived there, so the slingshot maneuver could be planned quite precisely.

The incredible precision of NASA's orbit calculations can be seen by the "

Pioneer Anomaly", which became too big to ignore by 1994. This was an unexpected velocity deviation of just 1km/h from the actual ≈15,000km/h, accumulated over a period of 10 years. It was eventually traced to the infinitessimal recoil from infra-red photons bouncing off the spacecraft's body!

Wikipedia says that the Planetary segment of the

Voyager 2 mission was 12 years & 1 month long. However, it's Interstellar mission has continued beyond that, for another 25 years (so far).

So I suspect that the 12 years Timey recalls has to do with the planned=actual duration of the initial planetary survey mission, ie the initial part of the schedule.