Radiocarbon calibration

Radiocarbon dating measurements produce ages in "radiocarbon years", which must be converted to calendar ages by a process called calibration. Calibration is needed because the atmospheric ¹⁴
C:¹²
C ratio, which is a key element in calculating radiocarbon ages, has not been constant historically.^[1]

Willard Libby, the inventor of radiocarbon dating, pointed out as early as 1955 the possibility that the ratio might have varied over time. Discrepancies began to be noted between measured ages and known historical dates for artefacts, and it became clear that a correction would need to be applied to radiocarbon ages to obtain calendar dates.^[2] Uncalibrated dates may be stated as "radiocarbon years ago", abbreviated "¹⁴
Cya".^[3]

The term Before Present (BP) is established for reporting dates derived from radiocarbon analysis, where "present" is 1950. Uncalibrated dates are stated as "uncal BP",^[4] and calibrated (corrected) dates as "cal BP". Used alone, the term BP is ambiguous.

Similarly, BC or BCE means 'Before Christ' or 'Before Current Era', and cal BC is used accordingly.