system
Given the following post, respond as if you're commenting on Reddit:
human
Does gravity affect the measurement of time or the actual passage of time itself?

There's are two cars on Earth that are placed on treadmills. They both accelerate to the same speed and hold the speed indefinitely. One of the treadmills is then transported to Jupiter. Assume all environmental parameters between the two sites are the same, same car/air density/etc. Let's also assume communication between the two cars are instantaneous. Once the treadmill lands on Jupiter, both treadmills are stopped and the cars shoot out at whatever speed they were traveling at prior. After 1 hour as measured on earth, both cars are instructed to stop immediately. Has the car on Jupiter gone a different distance because time passes at a different rate or that its speed was different as time was measured at a different rate.
gpt
Forget about instantaneous communication (which can't be considered consistent with relativity anyway) and treadmills and cars. You can only compare clocks when they are right next to each other. So suppose someone takes an extended vacation to Jupiter while someone else stays back on Earth. They synch their clocks when they separate. The person on Jupiter eventually comes back. When the two clocks reunite, they stop them and compare readings. The clock that went to Jupiter reads a lower elapsed time.

(Now in this example, the relative difference will not be very much at all. But you can replace Jupiter with your favorite supermassive black hole and have the vacationer orbit the black hole for some period before coming back. If the orbit is close enough and the mass of the black hole large enough, the relative difference between the two clocks at reunion can be very large, like the difference between weeks and years.)

Time dilation is not illusory or some contrived consequence of how we measure time. It is real and describes actual differences in elapsed time.