Again, you are not getting it.
These
known half-lives of radioactive isotopes…where the time required for half of the atoms of a particular radioactive substance to decay, where these decay rates are predictable (
known)…allows scientists to measure the quantity of the remaining unstable atoms left in a rock (for example) and compare it to the quantity of related stable atoms in the rock, and determine/estimate the amount of time that has passed since that rock was formed.
I get it just fine. But my faith isn’t so weak and frail that it depends upon the universe not being billions of years old.