Rapidly improving technologies have provided increasingly precise and widely applicable means for measuring geologic time. Whilst the ability to resolve age continues to improve, the absolute accuracy will remain limited by sources of systematic error that loom ever larger in comparison. The decay constants that characterize radioactive decay rates, and govern the relationship between isotopic data and their corresponding radioisotopic ages, are inexactly known. In consequence, the accuracy of some of the most precise dating methods, such as the 40Ar/39Ar technique, may be an order of magnitude or more worse than their precision. Scientific inquiries requiring comparison of ages determined by different radioisotopic systems, such as thermochronology or early Solar System evolution, are most impacted by these accuracy-limiting effects. Amelioration of the condition is straightforward (though not necessarily easy), but requires shifting research priorities and more interdisciplinary interaction.