The starting isotope is called the parent and the end-product is called the daughter.
It is possible to measure the ratio of the different radioactive parent isotopes and their daughter isotopes in a rock, but the ratios are not dates or ages.
The dates must be inferred based on assumptions about the ratios.
If certain things are known, it is possible to calculate the amount of time since the parent isotope began to decay.
For example, if you began with 1 gram of carbon-14, after 5,730 years you would be left with 0.50 g and only 0.25 g after 11,460 years.
Relative ages are assigned to rocks based on the idea that rock layers lower in the strata were deposited before rock layers that are higher.
Creationists do not necessarily disagree with this concept, but it can only be applied to layers that are found in one location and/or can be determined to have been deposited in a continuous layer over a very wide area.The reason this age may not be a true age—even though it is commonly called an absolute age—is that it is based on several crucial assumptions.Most radiometric dating techniques must make three assumptions: The major problem with the first assumption is that there is no way to prove that the decay rate was not different at some point in the past.A fear of God and reverence for His Word is the beginning of wisdom.Starting with the Bible and developing a model for dating events in earth history will lead us to the truth.Some of the common isotope pairs used are K-Ar, Rb-Sr, Pb-Pb, and U-Pb.