Radiocarbon dating rests on the decay of an isotope formed in the upper atmosphere, and its early results assumed that atmospheric concentration had been constant. It has not been. Solar activity, the carbon released by oceans and, more recently, fossil fuel burning have all altered it, so a raw radiocarbon measurement is not a calendar date but a value that must be translated into one. Calibration curves built from tree rings, corals and lake sediments perform that translation. The consequence is that a single measurement can correspond to more than one calendar range, and that improvements to the curve silently revise dates published decades earlier. Archaeological chronologies are provisional in a way that a date printed with an error bar does not convey.