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.
Uniformitarian geologists use so-called dating methods to determine the ages of the surrounding rocks.
Certain types of rocks, especially those that form from magma (igneous), contain radioactive isotopes of different elements.
If any of these three conditions is not accurately known, the hourglass will give an inaccurate measure of time.
Radiometric dating is based on the fact that radioactive isotopes decay to form isotopes of different elements.
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.
However, there are many methods that can be used to determine the age of the earth or other objects.
The textbooks focus on relative dating, based on the layering of the rocks, and radiometric dating.
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.