Ab urbe condita or Anno urbis conditae (Latin pronunciation: [ab ˈʊrbɛ ˈkɔndɪtaː], abbreviated: A.U.C. or AUC) is a convention that was used in antiquity and by classical historians to refer to a given year in Ancient Rome. Ab urbe condita literally means "from the founding of the City" and anno urbis conditae means "in the year since the City's founding". Therefore, the traditional date of the foundation of Rome, 753 BC, would be written 1 AUC. 1 AD, or 1 CE, would be 754 AUC. The foundation of the Empire, 27 BC or 27 BCE, would be 726 AUC.
Its use was more common in the Renaissance, when editors sometimes added AUC to Roman manuscripts they published, giving the false impression that the convention was commonly used in antiquity. In reality, the dominant method of identifying Roman years in Roman times was to name the two consuls who held office that year. In late antiquity, regnal years were also in use, as was the Diocletian era in Roman Egypt after AD 293, and in the Byzantine Empire after AD 537, following a decree by Justinian.
The traditional date for the founding of Rome, 21 April 753 BC, is due to Marcus Terentius Varro (1st century BC). Varro may have used the consular list with its mistakes and called the year of the first consuls "245 ab urbe condita", accepting the 244-year interval from Dionysius of Halicarnassus for the kings after the foundation of Rome. The correctness of Varro's calculation has not been confirmed, but it is still used worldwide.
From Emperor Claudius (ruled 41–54) onwards, Varro's calculation superseded other contemporary calculations.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 honour of the city's anniversary, in 48 AD, 800 years after the founding of the city. Hadrian and Antoninus Pius held similar celebrations, in 121 and 147/148 respectively.
In 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 "Year 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 Saeculum Novum.
The Anno Domini (AD) year numbering was developed by a monk named Dionysius Exiguus in Rome in 525, 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. In his Easter table, the year 532 was equated with the regnal year 248 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 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 …"). Blackburn and Holford-Strevens review interpretations of Dionysius which place the Incarnation in 2 BC, 1 BC, or AD 1.
It was later calculated (from the historical record of the succession of Roman consuls) that the year AD 1 corresponds to 754 AUC, based on Varro's epoch. Thus,