Because the “present” time changes, standard practice is to use 1 January 1950 as the commencement date of the age scale, reflecting the origin of practical radiocarbon dating in the 1950s. The abbreviation “BP” has alternatively been interpreted as “Before Physics”;[1] that is, before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable.

Source: Before Present – Wikipedia