Isotopic dating using long-lived radionuclides is used to estimate ages of rocks pulled from strata. Closed system behavior is assumed unless discordant results are noted. Concordant or discordant results do not guarantee the system is closed or open. The calculated age without a test of openness/closeness of the system has no merit. A systematic method of determining the age of a multilayered rock structure having open system behavior is proposed. When rock atomic loss constants for different dating methods are similar and much greater than the decay constants, the apparent closed system ages will be similar. A discussion is given as to how much movement of radioisotopes and their progeny from one layer to the next is allowed in order for closed system results to be reliable. The maximum datable age is defined.