An examination of sedimentary rocks worldwide shows a striking consistency with the unimaginably massive Flood that wiped out whole environments.
It is founded on unprovable assumptions such as 1) there has been no contamination and 2) the decay rate has remained constant.
So, we have a “clock” which starts ticking the moment something dies.
Obviously, this works only for things which were once living.
However, to read any clock accurately we must know where the clock was set at the beginning.
Most people think that radioactive dating has proven the earth is billions of years old.