Despite the fact that there are many scientific problems with radiometric dating, there is a more significant problem.
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.
Recent research by a team of creation scientists known as the RATE (arth) group has demonstrated the unreliability of radiometric dating techniques.
Even the use of isochron dating, which is supposed to eliminate some initial condition assumptions, produces dates that are not reliable.
There is also a difference in the timescale used to explain the layers.
Determining the relative age of a rock layer is based on the assumption that you know the ages of the rocks surrounding it.
Some of the common isotope pairs used are K-Ar, Rb-Sr, Pb-Pb, and U-Pb.