Anno Domini. The Christian era in the Gregorian calendar, starting from the year ad 1 as the calculated and back‐projected year in which Christ was believed to have been born. Widely used in western societies. The use of lower‐case initials is often taken to indicate that the date has been calculated from a radiocarbon determination by subtracting 1950 from the radiocarbon age. The use of upper‐case letters denotes a historical date or a calibrated radiocarbon age in calen‐ dar years.