Before Present
calendar era
Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date (epoch) of the age scale, with 1950 being labelled as the "standard year". The abbreviation "BP" has been interpreted retrospectively as "Before Physics", which refers to the time before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, which scientists must account for when using radiocarbon dating for dates of origin that may fall after this year.
In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating; the alternative notation "RCYBP" stands for the explicit "radio carbon years before present".
Usage
The BP scale is sometimes used for dates established by means other than radiocarbon dating, such as stratigraphy. This usage differs from the recommendation by van der Plicht & Hogg, followed by the Quaternary Science Reviews, both of which recommended that publications use the unit "a" (for "annum", Latin for "year") and reserve the term "BP" for radiocarbon estimations.
Some archaeologists use the lowercase letters bp, bc and ad as terminology for uncalibrated dates for these eras.
The Centre for Ice and Climate at the University of Copenhagen instead uses the unambiguous "b2k", for "years before 2000 AD", often in combination with the Greenland Ice Core Chronology 2005 (GICC05) time scale.
Some authors who use the YBP dating format also use "YAP" ("years after present") to denote years after 1950.
SI prefixes
SI prefix multipliers may be used to express larger periods of time, e.g., ka BP (thousand years BP), Ma BP (million years BP) and many others.
The public source identifies “Before Present” as calendar era. This brief keeps that definition visible, then builds a research path around Before, Present and calendar.
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A short description can identify a subject without explaining its stakes. For “Before Present”, the useful work is to connect “calendar era” to the records capable of establishing context and consequence.
Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated Jul 26, 2026. The linked authority identifier is Q503142. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1950, 2000 and 2005.
The absence of detail may reflect summary conventions rather than a lack of surviving documentation. The source lead contains qualifying language; that uncertainty should survive quotation, summary and reuse. Authority statements aid reconciliation but still require their own references, qualifiers and ranks to be checked.
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This entry incorporates text from “Before Present” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.