In the 70's: If this sort of thing interests you, you should find the journal Radiocarbon and read one of the issues devoted to calibration.For example, see Radiocarbon 46,1029 (2005), which has a calibration curve that goes back 26,000 years.When this was first done, it turned out that carbon dating had been giving too-young dates for early civilizations.
(Specifically, neutrons hit nitrogen-14 atoms and transmute them to carbon.) Land plants, such as trees, get their carbon from carbon dioxide in the air. The same is true of any creature that gets its carbon by eating such plants. Suppose such a creature dies, and the body is preserved.
If you hear of a carbon dating up in the millions of years, you're hearing a confused report. Second, they rarely contain any of the original carbon.
We can't date oil paints, because their oil is "old" carbon from petroleum. And third, it is common to soak new-found fossils in a preservative, such as shellac.
Then, we have wood for which we know the right answer.
So, carbon dating has been calibrated against the rings of California bristlecone pines, and Irish bog oaks, and the like.