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.Despite the fact that there are many scientific problems with radiometric dating, there is a more significant problem.
Even the use of isochron dating, which is supposed to eliminate some initial condition assumptions, produces dates that are not reliable.
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.
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.
Far from being data, these dates are actually interpretations of the data.