Common dating method radioisotope
The formula below is a proper model that admits the possibility that some daughter isotope was present when the rock formed: where D is the amount of daughter isotope present at start.
As any first-year student of algebra soon learns, a single equation with two unknown variables cannot be solved.
In fact, the above formula is far too simple, because it assumes that the amount of daughter isotope was zero at start.
The proportion of argon to radioactive potassium in the sample today is observable, and the decay constant of potassium is readily calculable by measuring the amount of argon produced from the decay of K after a specified time.
But the age of the rock and the proportion of argon to radio-potassium in the sample originally are not observable.
One possibility for the accelerated decay comes with the possibility of variable speed of light.