Years before present (or ybp for short) is a term used to describe a rough estimation of time when something occurred previously. As it is a rough estimation, often using an implied years per generation (of 25 to 33), it is a way to specify the time relative to the current date when read. That way, it moves as the current date moves. Often the estimates are accurate to within a quarter century (or generation), at most, and sometimes only a century or half millennium. The term is most often used in genetic genealogy and borrowed from the population genetics community. More accurately, the value associated with the term should be calculated from the birth year of the individual being considered and the "n generations" back from then. But this precision is often beyond the accuracy of the estimate.