Evolutionists assume that the rate of cosmic bombardment of the atmosphere has always remained constant and that the rate of decay has remained constant.
Scientists place great faith in this dating method, and yet more than 50% of radiocarbon dates from geological and archaeological samples of northeastern North America have been deemed unacceptable after investigation.
It's assumed that the clock was set to zero when the study material was formed.
This requires that only the parent isotope be initially present or that the amount of daughter isotope present at the beginning is known so that it can be subtracted.
It is assumed that we are dealing with a closed system—no loss of either parent or daughter elements has occurred since the study material formed.