The calendar era before and under the Roman kings is uncertain but dating by regnal years was common in antiquity. Under the Roman Republic, from 509BC, years were most commonly described in terms of their reigning ordinary consuls. (Temporary and honorary consuls were sometimes elected or appointed but were not used in dating.) Consular lists were displayed on the public calendars. After the institution of the Roman Empire, regnal dates based on the emperors' terms in office became more common. Some historians of the later republic and early imperial eras dated from the legendary founding of the city of Rome ( or ). Varro's date for this was 753BC but other writers used different dates, varying by several decades. Such dating was, however, never widespread. After the consuls waned in importance, most Roman dating was regnal. or followed Diocletian's 15-year Indiction tax cycle. These cycles were not distinguished, however, so that "year 2 of the indiction" may refer to any of 298, 313, 328, &c. The Orthodox subjects of the Byzantine Empire used various Christian eras, including those based on Diocletian's persecutions, Christ's incarnation, and the supposed age of the world.
