The first radiocarbon dates reported had their ages calculated to the nearest year, expressed in years before present (BP). It was soon apparent that the meaning of BP would change every year and that one would need to know the date of the analysis in order to understand the age of the sample. To avoid confusion, an international convention established that the year A.D. 1950 would be adopted as the reference point for the expression BP. Thus, BP means years before A.D. 1950.

Some people continue to express radiocarbon dates in relation to the calendar by subtracting 1950 from the reported age. This practice is incorrect, because it is now known that radiocarbon years are not equivalent to calendar years. To express a radiocarbon date in calendar years it must be normalized, corrected as needed for reservoir effects, and calibrated.