The claimed “fact” that decay rates have always been constant is actually an inference based on a uniformitarian assumption.It is true that radioisotope decay rates are stable today and are not largely affected by external conditions like change in temperature and pressure, but that does not mean that the rate has always been constant.Some of the common isotope pairs used are K-Ar, Rb-Sr, Pb-Pb, and U-Pb.Carbon-14 dating is another common technique, but it can only be used on carbon-containing things that were once alive.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.
Determining the relative age of a rock layer is based on the assumption that you know the ages of the rocks surrounding it.
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.
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.