Note that, contrary to a popular misconception, carbon dating is not used to date rocks at millions of years old.Before we get into the details of how radiometric dating methods are used, we need to review some preliminary concepts from chemistry.These two radiocarbon dating methods use modern standards such as oxalic acid and other reference materials.Although both radiocarbon dating methods produce high-quality results, they are fundamentally different in principle.
The two techniques are used primarily in determining carbon 14 content of archaeological artifacts and geological samples.
Since that decays and the other $\ce$-isotopes don't, over the years, the percentage of $^\ce$ is getting lower and lower at a constant rate (you can calculate that rate with the half-time of $^\ce$).
Now when you measure the relative amount of $^\ce$ in a skeleton, you know since when it hasn't eaten anymore, so you know how old it is.
I have tried here to answer some of the frequently asked questions that I receive from students via email, as well as providing some basic information about scientific dating methods.
"Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure.