Because samples get older as the years go by all radiocarbon ages are referred to AD 1950 as "present". Of course it is only for very precise dates that this is more than a trivial consideration, but the situation will gradually change in the future. It is less confusing to regard BP as meaning "before physics" - AD 1950 being the year in which radiocarbon dates began to be published. In radiocarbon terminology "BP" means and uncalibrated age, with "cal BP" indicating calibration.