rationalskepticism.org

I'm trying to understand how radiometric dating accounts for the time lag between the synthesis of radioisotopes and their subsequent deposition on proto-Earth. Assuming that radioisotopes begin decaying soon after they are formed in supernovae, then it would seem that radiometric dating indicates when the isotopes began decaying rather than the age of Earth in which are found today. If all radiometric dating on Earth points to a common starting point, then that could simply mean they all came from the same source supernova. If the time between radioisotope synthesis and the formation of our solar system is very small, then the age of isotopes is a reasonable estimate of the age of Earth. But if time between the synthesis of the isotopes and the formation of Earth is a significant fraction of the age of the isotopes, then Earth would be younger. But how this lag would be determined is not obvious. Please clarify my understanding.