ab urbe condita

{{see also|Founding of Rome#Chronological disagreements}}

Prior to the Roman state's adoption of the Varronian chronology – created by Titus Pomponius Atticus and Marcus Terentius Varro – there were many different dates posited for when the city was founded. This state of confusion required picking a canonical founding date for one to use an AUC date. The Varronian chronology, constructed from fragmentary sources and demonstrably about four years off of absolute events {{circa|340 BC}},{{Cite book |last=Forsythe |first=Gary |title=A critical history of early Rome |date=2005 |publisher=University of California Press |isbn=978-0-520-94029-1 |location=Berkeley |oclc=70728478 |page=279 }} placed the founding of the city on 21 April 753 BC. This date, likely arrived at by mechanical calculation but accepted with a variance of one year by the Augustan-era {{lang|la|fasti Capitolini}}, has become the traditional date.{{Cite book |last=Cornell |first=Tim |title=The beginnings of Rome |date=1995 |publisher=Routledge |isbn=0-415-01596-0 |location=London |oclc=31515793 |page=73 }} Varro likely arrived at 753 BC by counting seven generations of 35 years from his date for the founding of the republic in 509 BC.

From the time of Claudius ({{Reigned|AD 41|51}}) onward, this calculation superseded other contemporary calculations. Celebrating the anniversary of the city became part of imperial propaganda. Claudius was the first to hold magnificent celebrations in honor of the anniversary of the city, in AD 47,{{cite book |last1=Tacitus |first1=Cornelius |author1-link=Tacitus |editor1-last=Furneaux |editor1-first=Henry |editor1-link=Henry Furneaux |title=Annals XI|publisher=Clarendon Press |location=Oxford |page=17|edition=1907|quote=ludi saeculares octingentesimo post Romam conditam|lang=la}}{{cite web |last1=Bilynskyj Dunning |first1=Susan |title=saeculum |url=https://oxfordre.com/classics/display/10.1093/acrefore/9780199381135.001.0001/acrefore-9780199381135-e-8233 |website=Oxford Research Encyclopedia of Classics |publisher=Oxford University Press |doi=10.1093/acrefore/9780199381135.013.8233 |date=20 November 2017|isbn=978-0-19-938113-5 }} the eight hundredth year from the founding of the city.{{cite book |last1=Hobler |first1=Francis |title=Records of Roman history, from Cnaeus Pompeius to Tiberius Constantinus, as exhibited on the Roman coins |date=1860 |publisher=John Bowyer Nichols |location=London |page=222}} Hadrian, in AD 121, and Antoninus Pius, in AD 147 and AD 148, held similar celebrations respectively.

In AD 248, Philip the Arab celebrated Rome's first millennium, together with Ludi saeculares for Rome's alleged tenth saeculum. Coins from his reign commemorate the celebrations. A coin by a contender for the imperial throne, Pacatianus, explicitly states "[y]ear one thousand and first", which is an indication that the citizens of the empire had a sense of the beginning of a new era, a Sæculum Novum.

{{see|Calendar era}}

The Anno Domini (AD) year numbering was developed by a monk named Dionysius Exiguus in Rome in {{auc|525|AD|main=greg|alinks=off}}, as a result of his work on calculating the date of Easter. Dionysius did not use the AUC convention, but instead based his calculations on the Diocletian era. This convention had been in use since AD 293, the year of the tetrarchy, as it became impractical to use regnal years of the current emperor.{{Cite journal |last=Thomas |first=J. David |date=January 1971 |title=On Dating by Regnal Years of Diocletian, Maximian and the Caesars |url=https://www.brepolsonline.net/doi/10.1484/J.CDE.2.308234 |journal=Chronique d'Égypte |language=fr |volume=46 |issue=91 |pages=173–179 |doi=10.1484/J.CDE.2.308234 |issn=0009-6067}} In his Easter table, the year {{auc|532|AD|main=greg|alinks=off}} was equated with the 248th regnal year of Diocletian. The table counted the years starting from the presumed birth of Christ, rather than the accession of the emperor Diocletian on 20 November AD 284 or, as stated by Dionysius: "sed magis elegimus ab incarnatione Domini nostri Jesu Christi annorum tempora praenotare" ("but rather we choose to name the times of the years from the incarnation of our Lord Jesus Christ").Migne, Jacques-Paul. 1865. [https://books.google.com/books?id=LcEYeL-4ZuEC&dq=%22in+hoc+tomo+LXVII&pg=PP10 Liber de Paschate] (Patrologia Latina 67), p. 481, § XX, note f Blackburn and Holford-Strevens review interpretations of Dionysius which place the Incarnation in 2 BC, 1 BC, or AD 1.Blackburn, B. & Holford-Strevens, L, The Oxford Companion to the Year (Oxford University Press, 2003 corrected reprinting, originally 1999), pp. 778–780.

The year AD 1 corresponds to AUC 754, based on the epoch of Varro. Thus:

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