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.
As the name suggests, fossil fuel is old, and no longer contains C14.
Both of these man-made changes are a nuisance to carbon dating.
Historians don't have "right answers" for really old things.
However, carbon dating has done well on young material like the Dead Sea Scrolls, the Minoan ruins, and acacia wood from the tomb of the pharoah Zoser.
We know (from other measurements) that the Sun hasn't fluctuated by more than 10 percent in the last million years.
However, even this small an adjustment was a bit of a shock. It is produced in the upper atmosphere by radiation from the sun.(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.Old samples contain much less C14, so the measured date of older samples is strongly affected by even small amounts of contamination.The third kind are dates which were measured before the 1970's.On the Web, you could visit a dating laboratory, visit a dating service, read an encyclopedia entry or read a critique.