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between sub-annually and annually resolved records available from tree rings and ice cores, in addition to those from historical documents.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 BC 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.

Baillie M.G.L., 1995 – A slice through time: Dendrochronology and precision dating. B.T. Batsford, London (UK).

Bateman M.D., 2015 – The application of luminescence dating in sea-level studies. In: Handbook of Sea-Level Research. Shennan I., Long A.J. & Horton B.P. (eds.). John Wiley & Sons, Chichester, 404-417. <https://doi.org/10.1002/9781118452547.ch27>

Brehm N., Christl M., Knowles T.D.J., Casanova E., Evershed R.P., Adolphi F., Muscheler R., Synal H.-A., Mekhaldi F., Paleari C.I., Leuschner H.-H., Bayliss A., Nicolussi K., Pichler T., Schlüchte, C., Pearson C.L., Salzer M.W., Fonti P., Nievergelt D., Hantemirov R., Brown D.M., Usoskin I. & Wacker L., 2022 – Tree-rings reveal two strong solar proton events in 7176 and 5259 BCE. Nature Communications, 13, 1196: 1-8. <https://doi.org/10.1038/s41467-022-28804-9>

Bronk Ramsey C., 2009 – Bayesian Analysis of Radiocarbon Dates. Radiocarbon, 51 (1): 337-360. <https://doi.org/10.1017/S0033822200033865>

Bronk Ramsey C., Adolphi F., Austin W., Bard E., Bayliss A., Blaauw M., Cheng H., Edwards R.L., Friedrich M., Heaton T., Hogg A., Hua Q., Hughen K., Kromer B., Manning S., Muscheler R., Palmer J., Pearson C., Reimer P., Reimer R., Richards D., Scott M., Southon J., Turney C. & Wacker L., 2024 – Development of the Intcal Database. Radiocarbon, 66 (6): 1852-1868. <https://doi.org/10.1017/RDC.2023.53>

Büntgen U. & Oppenheimer C, 2020 – The importance of “year zero” in interdisciplinary studies of climate and history. Proceedings of the National Academy of Sciences of the United States of America, 117 (52): 32845-32847. <https://doi.org/10.1073/pnas.2018103117>

Butler P.G., Wanamaker A.D., Scourse J.D., Richardson C.A. & Reynolds D.J., 2013 – Variability of marine climate on the North Icelandic Shelf in a 1357-year proxy archive based on growth increments in the bivalve Arctica islandica. Palaeogeography, Palaeoclimatology, Palaeoecology, 373: 141-151. <https://doi.org/10.1016/j.palaeo.2012.01.016 >

Declercq G., 2002 – Dionysius Exiguus and the Introduction of the Christian Era. Sacris Erudiri, 41: 165-246. <https://doi.org/10.1484/J.SE.2.300491>

Duller G.A.T., 2011 – What date is it? Should there be an agreed datum for luminescence ages? Ancient TL, 29 (1): 1-3. <https://doi.org/10.26034/la.atl.2011.442>

Emiliani C., 1993 – Calendar reform. Nature 366: 716. <https://doi.org/10.1038/366716b0>

Emiliani C., 1995 – Counting time. Nature 373: 278. <https://doi.org/10.1038/373278c0>

Fahrni S.M., Southon J., Fuller B.T., Park J., Friedrich M., Muscheler R., Wacker L. & Taylor R.E., 2020 – Single-Year German oak

and Californian Bristlecone Pine

Hallstatt Plateau from 856 BC to 626 BC. Radiocarbon, 62 (4): 919-937. <https://doi.org/10.1017/RDC.2020.16>

Flickinger R.C., 1931 – Who Were the Roman Consuls for the Year Zero? The Classical Journal 26 (5): 337-339. <https://www.jstor.org/stable/3289903>

Friedrich R., Kromer B., Wacker L., Olsen J., Remmele S., Lindauer S., Land A. & Pearson C., 2020 – A New Annual C Dataset for Calibrating the Thera Eruption. Radiocarbon, 62 (4): 953-961. <https://doi.org/10.1017/RDC.2020.33>

Fritts H.C., 1976 –Tree Rings and Climate. Academic Press, New York (NY).

Gibbard P.L. & Head M.J., 2020 – The Quaternary Period. In: Geologic Time Scale 2020. Gradstein F.M., Ogg J.G., Schmitz M.D. & Ogg G.M. (eds.). Elsevier, 2: 1217-1255. <https://doi.org/10.1016/B978-0-12-824360-2.00030-9>

Godwin H., 1962 – Half-life of Radiocarbon. Nature, 195: 984. <https://doi.org/10.1038/195984a0>

Gould S.J., 2011 – Questioning the millennium. Harvard University Press, Cambridge (MA).

Grün R., 2008 – Editorial. Quaternary Geochronology, 3 (1-2): 1. <https://doi.org/10.1016/j.quageo.2007.09.001>

Holden N.E., Bonardi M.L., De Bievre P., Renne P.R. & Villa I.M., 2011 – IUPAC-IUGS Common Definition and Convention on the Use of the Year as a Derived Unit of Time. Episodes, 34 (1): 39-40. <https://doi.org/10.18814/epiiugs/2011/v34i1/006>

Hua Q, Barbetti M, Levchenko VA, D’Arrigo R.D., Buckley BM and Smith AM, 2012 – Monsoonal influence on Southern Hemisphere 14CO2. Geophysical Research Letters, 39 (19): 1-5. <https://doi.org/10.1029/2012GL052971>

Hua Q., Barbetti M. & Rakowski A.Z., 2013 – Atmospheric Radiocarbon for the Period 1950-2010. Radiocarbon, 55 (4): 2059-2072. <https://doi.org/10.2458/azu_js_rc.v55i2.16177>

International Organization for Standardization, 2004 – International Standard ISO 8601. Data elements and interchange formats. Information interchange. Representation of dates and times. 3rd ed. ISO copyright office, Geneva (CH).

Kalliokoski M., Guðmundsdóttir E.R., Wastegård S., Jokinen S. & Saarinen T., 2023 – A Holocene tephrochronological framework for Finland. Quaternary Science Reviews, 312: 1-18. <https://doi.org/10.1016/j.quascirev.2023.108173>

Kukla G., 1994 – Counting time. Nature, 372: 124. <https://doi.org/10.1038/372124d0>

Lambe S., 2024 – Waste of Time? Why do we use BC/AD – and should we keep them? History Today, 74: 22-24.

Larsen D.J., Miller G.H., Geirsdóttir Á. & Thordarson T., 2011 – A 3000-year varved record of glacier activity and climate change from the proglacial lake Hvítárvatn, Iceland. Quaternary Science Reviews, 30 (19-20): 2715-2731. <https://doi.org/10.1016/j.quascirev.2011.05.026>

Lund K., 1999 – International standard for denotation of calendar time. Production Planning & Control, 10: 815-817. <https://doi.org/10.1080/095372899232641>

Luterbacher J., Werner J.P., Smerdon J.E., Fernandez-Donado L., Gonzalez-Rouco F.J., Barriopedro D., Ljungqvist F.C., Buentgen U., Zorita E., Wagner S., Esper J., Mccarroll D., Toreti A., Frank D., Jungclaus J.H., Barriendos M., Bertolin C., Bothe O., Brazdil R., Camuffo D., Dobrovolny P., Gagen M., Garica-Bustamante E., Ge Q., Gomez-Navarro J.J., Guiot J., Hao Z., Hegerl G.C., Holmgren K., Klimenko V.V., Martin-Chivelet J., Pfister C., Roberts N., Schindler A., Schurer A., Solomina O., Gunten L., Wahl E. Wanner H., Wetter O., Xoplaki E., Yuan N., Zanchettin D., Zhang H & Zerefos C., 2016 – European summer temperatures since Roman times. Environmental Research Letters, 11 (2): 1-12. <http://dx.doi.org/10.1088/1748-9326/11/2/024001>

Maczkowski A., Pearson C., Francuz J., Giagkoulis T., Szidat S., Wacker L., Bolliger M., Kotsakis K. & Hafner A., 2024 – Absolute dating of the European Neolithic using the 5259 BC rapid 14C excursion. Nature Communications, 15, 4263. <https://doi.org/10.1038/s41467-024-48402-1>

Martínez G., Martínez G.A. & Owen L.A., 2023 – Human occupation, site formation, and chronostratigraphy of a mid-Holocene archaeological site at the eastern Pampa-Patagonia transition, Argentina. Quaternary Research, 114: 52-68. <https://doi.org/10.1017/qua.2023.8>

McConnell J.R., Sigl M., Plunkett G., Burke A., Kim W.M., Raible C.C., Wilson A.I., Manning J.G., Ludlow F., Chellman N.J., Innes H.M., Yang Z., Larsen J.F., Schaefer J.R., Kipfstuhl S., Mojtabavi S., Wilhelms F., Opel T., Meyer H. & Steffensen J.P., 2020 – Extreme climate after massive eruption of Alaska’s Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom. Proceedings of the National Academy of Sciences of the United States of America, 117 (27): 15443-15449. <https://doi.org/10.1073/pnas.2002722117>

Miyake F., Panyushkina I.P., Jull A.J.T., Adolphi F., Brehm N., Helama S., Kanzawa K., Moriya T., Muscheler R., Nicolussi K., Oinonen M., Salzer M., Takeyama M., Tokanai F. & Wacker L., 2021 – A single-year cosmic ray event at 5410 BCE registered in 14C of tree rings. Geophysical Research Letters, 48 (11): 1-8. <https://doi.org/10.1029/2021GL093419>

Noller J.S., Sowers J., Colman S. & Pierce K., 2000 – Introduction to Quaternary geochronology. In: Quaternary Geochronology; Methods and Applications. Noller J.S., Sowers J.M. & Lettis W.R. (eds.). AGU Reference Shelf, Washington DC, USA, 4: 1-10. <https://doi.org/10.1029/RF004p0001>

Pearson C.L., Brewer P.W., Brown D., Heaton T.J., Hodgins G.W.L., Jull A.J.T., Lange T. & Salzer M.W., 2018 – Annual radiocarbon record indicates 16th century BCE date for the Thera eruption. Science Advances, 4 (8): 1-7. <https://www.science.org/doi/10.1126/sciadv.aar8241>

Pearson C., Salzer M., Wacker L., Brewer P., Sookdeo A. & Kuniholm P., 2020 – Securing timelines in the ancient Mediterranean using multiproxy annual tree-ring data. Proceedings of the National Academy of Sciences of the United States of America, 117 (15): 8410-8415. <https://doi.org/10.1073/pnas.1917445117>

Pearson C., Sigl M., Burke A., Davies S., Kurbatov A., Severi M., Cole-Dai J., Innes H., Albert P.G. & Helmick M., 2022 – Geochemical ice-core constraints on the timing and climatic impact of Aniakchak II (1628 BCE) and Thera (Minoan) volcanic eruptions. PNAS Nexus, 1 (2): 1-12. <https://doi.org/10.1093/pnasnexus/pgac048>

Randall J., 2000 – [Comment on “Which one is correct, 2000 or 2001? How about 1995?”]. Eos, 81 (45): 532. <https://doi.org/10.1029/EO081i045p00532-02>

Rasmussen S.O., Andersen K.K., Svensson A.M., Steffensen J.P., Vinther B.M., Clausen H.B., Siggaard-Andersen M.-L., Johnsen S.J., Larsen L.B., Dahl-Jensen D., Bigler M., Röthlisberger R., Fischer H., Goto-Azuma K., Hansson M.E. & Ruthon U., 2006 – A new Greenland ice core chronology for the last glacial termination. Journal of Geophysical Research, 111 (D6): 1-16. <https://doi.org/10.1029/2005JD006079>

Rasmussen S.O., Svensson A.M. & Vinther B.M., 2022 – Greenland Ice-Core Chronology 2005 (GICC05) annual layer depths for various Greenland ice cores [dataset bundled publication]. PANGAEA. <https://doi.org/10.1594/PANGAEA.943195> (Retrieved in March 3, 2025).

Rasmussen S.O., Dahl-Jensen D., Fischer H., Fuhrer K., Hansen S.B., Hansson M., Hvidberg C.S., Jonsell U., Kipfstuhl S., Ruth U., Schwander J., Siggaard-Andersen M.-L., Sinnl G., Steffensen J.P., Svensson A.M. & Vinther B.M., 2023 – Ice-core data used for the construction of the Greenland Ice-Core Chronology 2005 and 2021 (GICC05 and GICC21). Earth System Science Data, 15 (8): 3351-3364. <https://doi.org/10.5194/essd-15-3351-2023, 2023>

Regev J., Gadot Y., Uziel J., Chalaf O., Shalev Y., Roth H., Shalom N., Szanton N., Bocher E., Pearson C.L., Brown D.M., Mintz E., Regev L. & Boaretto E., 2024 – Radiocarbon chronology of Iron Age Jerusalem reveals calibration offsets and architectural developments. Proceedings of the National Academy of Sciences of the United States of America, 121 (19), e2321024121. <https://doi.org/10.1073/pnas.2321024121>

Reimer P., Austin W., Bard E., Bayliss A., Blackwell P., Bronk Ramsey C., Butzin M., Cheng H., Edwards R., Friedrich M., Grootes P., Guilderson T., Hajdas I., Heaton T., Hogg A., Hughen K., Kromer B., Manning S., Muscheler R., Palmer J., Pearson C., van der Plicht J., Reimer R., Richards D., Scott E., Southon J., Turney C., Wacker L., Adolphi F., Büntgen U., Capano M., Fahrni S., Fogtmann-Schulz A., Friedrich R., Köhler P., Kudsk S., Miyake F., Olsen J., Reinig F., Sakamoto M., Sookdeo A. & Talamo S., 2020 – The IntCal20 Northern Hemisphere radiocarbon age calibration curve (0-55 kcal BP). Radiocarbon, 62 (4): 725-757. <https://doi.org/10.1017/RDC.2020.41>

Reimer P.J., Brown T.A. & Reimer R.W., 2004 – Discussion: Reporting and Calibration of Post-Bomb 14C Data. Radiocarbon, 46 (3): 1299-1304. <https://doi.org/10.1017/S0033822200033154>

Reynolds D.J., Richardson C.A., Scourse J.D., Butler P.G., Hollyman P., Román-González A. & Hall I.R., 2017 – Reconstructing North Atlantic marine climate variability using an absolutely-dated sclerochronological network. Palaeogeography, Palaeoclimatology, Palaeoecology, 465 (Part B): 333-346. <https://doi.org/10.1016/j.palaeo.2016.08.006>

Rose J., 2007 – The use of time units in Quaternary Science Reviews. Quaternary Science Reviews, 26 (9-10): 1193. <https://doi.org/10.1016/j.quascirev.2007.04.002>

Sakurai H., Tokanai F., Miyake F., Horiuchi K., Masuda K., Miyahara H., Ohyama M., Sakamoto M., Mitsutani T. & Moriya T., 2020 – Prolonged production of 14C during the ~660 BCE solar proton event from Japanese tree rings. Scientific Reports, 10, 660. <https://ui.adsabs.harvard.edu/link_gateway/2020NatSR..10..660S/doi:10.1038/s41598-019-57273-2>

Sano M., Kimura K., Miyake F., Tokanai F. & Nakatsuka T., 2023 – Two new millennium-long tree-ring oxygen isotope chronologies (2349–1009 BCE and 1412–466 BCE) from Japan. Radiocarbon, 65 (3): 721-732. <https://doi.org/10.1017/RDC.2023.29>

Schulman E., 1956 – Dendroclimatic changes in Semiarid America. University of Arizona Press, Tucson (AZ).

Sigl M., Winstrup M., McConnell J.R., Welten K.C., Plunkett G., Ludlow F., Büntgen U., Caffee M., Chellman N., Dahl-Jensen D., Fischer H., Kipfstuhl S., Kostick C., Maselli O.J., Mekhaldi F., Mulvaney R., Muscheler R., Pasteris D.R., Pilcher J.R., Salzer M., Schüpbach S., Steffensen J.P., Vinther B.M. & Woodruff T.E., 2015 – Timing and climate forcing of volcanic eruptions for the past 2,500 years. Nature, 523: 543-549. <https://doi.org/10.1038/nature14565>

Sinnl G., Winstrup M., Erhardt T., Cook E., Jensen C.M., Svensson A., Vinther B.M., Muscheler R. & Rasmussen S.O., 2022 – A multi-ice-core, annual-layer-counted Greenland ice-core chronology for the last 3800 years: GICC21. Climate of the Past, 18 (5): 1125-1150. <https://doi.org/10.5194/cp-18-1125-2022>

Speer J.H., 2010 – Fundamentals of Tree-ring Research. The University of Arizona Press, Tucson (AZ).

Stange U., 2024 – The Anthropocene as a civil time unit. The Anthropocene Review, 11 (3): 550-569. doi: <https://doi.org/10.1177/20530196231204326>

Stavi I., Ragolsky G., Haiman M. & Porat N., 2021 – Ancient to recent-past runoff harvesting agriculture in the hyper-arid Arava Valley: OSL dating and insights. The Holocene, 31 (6): 1047-1054. <https://doi.org/10.1177/0959683621994641>

Stuiver M. & Pearson G.W., 1993 – High-precision bidecadal calibration of the radiocarbon time scale, AD 1950-500 BC and 2500-6000 BC. Radiocarbon, 35 (1): 1-23. <https://doi.org/10.1017/S0033822200013783>

Uusitalo J., Arppe L., Hackman T., Helama S., Kovaltsov G., Mielikäinen K., Mäkinen H., Nöjd P., Palonen V., Usoskin I. & Oinonen M., 2018 – Solar superstorm of AD 774 recorded subannually by Arctic tree rings. Nature Communications, 9, 3495. <https://doi.org/10.1038/s41467-018-05883-1>

van der Plicht J. & Hogg A, 2006 – A note on reporting radiocarbon. Quaternary Geochronology, 1 (4): 237-240. <https://doi.org/10.1016/j.quageo.2006.07.001>

Veronis G., 2000a – Which one is correct, 2000 or 2001? How about 1995? Eos, 81 (26): 290. <https://doi.org/10.1029/00EO00216>

Veronis G., 2000b – Reply [to “Comments on “Which one is correct, 2000 or 2001? How about 1995?”]. Eos, 81 (45): 532. <https://doi.org/10.1029/EO081i045p00532-03>

Walker M.J.C., 2005 – Quaternary Dating Methods. John Wiley & Sons, Chichester (UK).

Walker M., Head M.J., Berkelhammer M., Björck S., Cheng H., Cwynar L., Fisher D., Gkinis V., Long A., Lowe J., Newnham R., Rasmussen S.O. & Weiss H., 2018 – Formal ratification of the subdivision of the Holocene Series/Epoch (Quaternary System/Period): Two new Global Boundary Stratotype Sections and Points (GSSPs) and three new stages/subseries. Episodes, 41 (4): 213-223. <https://doi.org/10.18814/epiiugs/2018/018016>

Wilkins G.A., 2000 – The year with a name but without a number. Astronomy & Geophysics, 41 (6): 6.9. <https://doi.org/10.1093/astrog/41.6.6.9-a>

Winger R.M., 1936 – Zero and the calendar. The Scientific Monthly, 43 (4): 363-367.

Wolff E.W., 2007 – When is the ‘‘present’’? Quaternary Science Reviews, 26 (25-28): 3023-3024. <https://doi.org/10.1016/j. quascirev.2007.10.008>

Yang B., Qin C., Wang J., He M., Melvin T.M., Osborn T.J. & Briffa K.R., 2014 – A 3,500-year tree-ring record of annual precipitation on the northeastern Tibetan Plateau. Proceedings of the National Academy of Sciences of the United States of America, 111 (8): 2903-2908. <https://doi.org/10.1073/pnas.1319238111>

Yang B., Qin C., Bräuning A., Osborn T.J., Trouet V., Ljungqvist F.C., Esper J., Schneider L., Grießinger J., Büntgen U., Rossi S., Dong G., Yan M., Ning L., Wang J., Wang X., Wang S., Luterbacher J., Cook E.R. & Stenseth N.C., 2021 – Long-term decrease in Asian monsoon rainfall and abrupt climate change events over the past 6,700 years. Proceedings of the National Academy of Sciences of the United States of America, 118 (30): 1-7. <https://doi.org/10.1073/pnas.2102007118>