For dating of rocks & fossils, etc., on and in the Earth, it's irrelevant - they all have the same time dilation relative to external clocks because they share the Earth's proper time; but when external clocks are relevant (e.g. dating comparisons with meteorites), the time difference is too small to be significant. IIRC, the gravitational time dilation is larger than the rotational time dilation at the surface, and that's only estimated to be around 2.5 days over the Earth's existence so far.
If the time dilation was large enough to be significant, it would be taken into account in dating comparisons with extra-terrestrial rocks.
Those extra terrestrial rocks are moving with the same relative velocity as earth.
But notice those extraterrestrial rocks all date to an earlier age than earth, even if it all supposedly formed at the same approximate time from the same material.
Because they are on different curved trajectories and although like the airplane only slightly different than earths. Their clocks ticked even faster than earths did, making them appear older. EDIT: Outer orborts orbit the sun slower, so the affect on clocks would not be the same for material from far orbits. They would age slightly faster than earth rates, being in the same rough curved trajectory, but in slower orbits. So would date older, even when not. End edit.
You keep ignoring that the twin couldn’t tell his clocks slowed, even when they did. You understand that like those clocks on airplanes that slowed, when no person aboard it could tell by any clock on board, so earths clocks slow.
It doesn’t matter if they all slow at the same exact rate, the further back you go the faster they get, until it becomes exponential. You only think everything remains the same like the twin incorrectly believed the same thing.
Every rock sample on board that rocket ship would have slowed in decay as well. So that had he continued his journey for 6,000 years instead of just a few, you would believe those rocks were an exponential age older, basing their decay rate on a rate slowing as we speak. But in 6,000 years of the stationary twins time, those rocks would have aged 4+billion years to people on board the rocket ship.
So once we calculate for time dilation, we would find the rocks had in actuality only aged 6,000 years, not 4+ billion. You must use a sliding scale rate of decay that continues to increase as you calculate backwards. Any other way will always get the wrong answer.
Proper time? The earths proper time is slowing as we speak, you just can’t see it like the twin couldn’t see it. But I’ll tell you what, tell me how old the twin is traveling at 1/2 of c for 10 years, after living in the stationary frame for 20 years. And do it without time dilation corrections. The answer you get will be wrong.
Our devices say we are stationary, even when you know we are not.... yet you don’t believe we are stationary because our devices tell us we are, do you?