Turn cal BP ages into cal BC/AD (equivalent to cal BCE/CE). Negative ages indicate cal BC, positive ages cal AD. Since the Gregorian and Julian calendars do not include 0 BCAD (i.e., 31 December of 1 BC is followed by 1 January of AD 1), zero can be omitted. The years then go from -1 (i.e., 1 BC) to 1 AD. Other calendars, such as the astronomical one, do include zero.