An overview of the calendar year timelines used in quaternary science and human history, with special reference to sub-annual dating

The year 2025 celebrated the 1500th anniversary of the way we count years since and before the incarnation of Christ. The BC/AD timeline is used in natural and human sciences, with conversions to cal BP and b2k timelines, the latter two representing ages before AD 1950 and AD 2000. Though the timelines may seem established, there are more ways than one to count the calendar years, especially “before Christ” and “before present”. If not correctly addressed, the disparities may impair comparisons between sub-annually and annually resolved records available from tree rings and ice cores, in addition to those from historical documents. The offsets between the timelines range from 0.5 to 2.5 years, which may already be detrimental to determining the cause-effect relationships of short-term events such as volcanic eruptions; however, it is possible that the errors may accumulate if different labs and re-users of data repeat the misconceptions. For example, a tree-ring event dated to 1627.5 B using one timeline might be misassigned to 1626.5 BC or 1625.5 BC using other timelines. Such confusion may arise due to a “year zero” that is missing from the historical timeline, when the AD and BC years are replaced by positive and negative decimal numbers and when the timelines are converted to the Cartesian coordinate system, and/or due to the timelines used for boreal and austral growing seasons. A previously published formula to convert calendar dates to cal BP dates should be replaced by modified formulae tailored to sub-annual dating in this paper. Similar formulae are also provided for converting b2k ages. Developments in the Quaternary dating methods suggest that the number of disciplines topical to this discussion is likely to increase in the near future. It is emphasized that a clear reference to the particular timeline employed must be clarified in every single individual geoscientific/interdisciplinary study.