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.
The claimed “fact” that decay rates have always been constant is actually an inference based on a uniformitarian assumption.
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.
There are three main assumptions that must be made to accept radiometric dating methods.
These must be accepted on faith in uniformitarian and naturalistic frameworks.
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.Some of the common isotope pairs used are K-Ar, Rb-Sr, Pb-Pb, and U-Pb.