Radioactive decay of minerals, such as Uranium-238 and Carbon-14 decay in organisms spring to mind. The amount there is present points to a 4.5 billion year old Earth for Uranium, as there's around half Uranium present and its fissionable material, that coupled with the fact it has a 4.468 billion year half-life means the Earth must be that old.
https://en.wikipedia.org/wiki/Uranium-238
https://en.wikipedia.org/wiki/Carbon-14
https://en.wikipedia.org/wiki/Uranium-238
https://en.wikipedia.org/wiki/Carbon-14
"The first principle is that you must not fool yourself — and you are the easiest person to fool." - Richard P. Feynman
