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Exact dates in Roman chronology in the first century B.C. are determined by dead-reckoning backwards from the point at which the Roman civil calendar converges to the historical Julian calendar. Therefore we must first confirm that the Roman civil calendar does in fact correspond exactly to the Julian calendar in the first century A.D., since any error in this assumption will inevitably affect earlier conversions.

First, the fact that the Roman civil calendar was Julian in structure in the reign of Augustus is confirmed by several fasti, or calendars, that have been discovered which date from his reign and that of Tiberius, such as the Fasti Maffeani and the Fasti Praenestini. Hence it is necessary only to confirm the Julian equivalence of one Roman civil date in this period by external evidence, and to confirm the phase of the leap year cycle in the imperial age.

I have found three synchronisms for this period, apart from synchronisms to non-Roman (Julian) calendars:

I have not found any direct evidence nor any pair of synchronisms that allows us to infer the precise phase of the intercalary cycle in the first century A.D. The intervals between these synchronisms confirm the use of a quadrennial cycle, and that its phase was either the same as the Julian cycle or one year later. The Julian phase is directly attested in the second century A.D. by inscription CIL VIII 6979, which records a temple dedication on the day after the bissextile (leap day) in A.D. 168. The next attested bissextile day is A.D. 364 (Ammianus Marcellinus 26.1.7)

These facts prove that the Julian calendar which we use to date ancient events was the actual Roman civil calendar in the early empire.

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