In this specific case, perhaps the dates have been taken from two different sources and the author has not made the effort to convert them to a common format.

In this specific case, perhaps the dates have been taken from two different sources and the author has not made the effort to convert them to a common format. In general, dates "before present" are used for the dates determined from carbon 14 decay, which do not have an intrinsic relationship with any calendar. A prehistoric cheese that is 7000 years old must have been dated with carbon 14, so "before present" would be the original dating, not yet converted to the Christian calendar.