Raw radiocarbon measurements are usually reported as years "before present" (BP). This is the number of radiocarbon years before 1950, based on the assumption that the level of carbon-14 in the atmosphere has always been at the 1950 level. Because this is untrue in practice, radiocarbon years do not correspond exactly to calendar years. Today calibrated dates, that take into account these changes, are possible.
See radiocarbon dating for more information.
