Many different local calendars were used in the eastern and western parts of the Roman empire. The Julian calendar introduced about 44 BC was the official calendar of the Roman republic and empire and was used by Roman citizens, the army, and the imperial administration.
The Julian calendar could be used with any "era" or year count, but if it didn't use a starting year that made the leap years the same as in the official version then the dates could be off.
The official Roman year began on January First, but the Julian calendar and other calendars could start the year at any date. For example, medieval England used the Julian calendar, but started the year on The Feast of the Annunciation, March 25, from 1155 to 1752. March 25 was the first day of the year, and March 24 was the last day of the year.
It was common to call the year by the names of the regular consuls of the year. For example, 59 BC was called the consulship pf Caesar and Bibulus, but since Gaius Julius Caesar was more dominant it was often jokingly called the consulship of Julius and Caesar. So historians had to keep long lists of the successive consuls.
During the empire it was common to use the regnal year of the emperor or emperors, and Justinian made it mandatory in 537 AD.
Romans sometimes counted the years ab urbe condita or anno urbis conditae, AUC, since the founding of Rome. Emperor Claudius made the calculation of Marcus Terentius Varro official, officially dating the foundation to 21 April 753 BC.
Hellenistic Greeks often dated events by Olympiads, the first Olympiad being from 776 BC to 772 BC.
Diodorus Siculus dates the Persian invasion of Greece to 480 BC: "Calliades was archon in Athens, and the Romans made Spurius Cassius and Proculus Verginius Tricostus consuls, and the Eleians celebrated the Seventy-fifth Olympiad, that in which Astylus of Syracuse won the stadion. It was in this year that king Xerxes made his campaign against Greece."
Jerome, in his Latin translation of the Chronicle of Eusebius, dates the birth of Jesus Christ to year 3 of Olympiad 194, the 42nd year of the reign of the emperor Augustus, which equates to the year 2 BC.
Hellenistic Greeks often used the Seleucid era, or year of the Greeks, dating from 312/11 BC.
Christian chronologists often dated events from the Crucifixion of Christ or from his incarnation. The modern AD/BC system came into use in western Europe during and after the reign of Charlemagne.
The 15 year indiction cycle began for tax purposes about 300 AD. In a few decades non tax documents began being dated by the indicition cycle. In 537 AD emperor Justinian decreed that all dates must include the indiction.
Christians also often dated events from the biblical creation (anno mundi or Etos Kosmou) of the world. Scholars proposed - and often used in their chronicles - tens and even hundreds of different dates for the creation of the world. I myself once calculated several different dates of Creation.
Most dates for the Creation are either within a few centuries of the date of Creation in the Hebrew Calendar, 6 October 3761 BC, or else within a few centuries of the date of Creation in the eventual Byzantine Era adopted in 988 AD, dating Creation to 1 September 5509 BC. The Byzantine Era was first adopted by the Sixth Ecumenical Council in 680/81 AD and first officially used in the Quinisext Council of 692 AD or 6200 AM.
Thus the Hebrew year 5777 began on 2 October 2016, while the Byzantine year 7526 should begin on 1 September 2017.
Professor Fr. Arsenius John Baptist Vuibert (S.S.), a 19th-century historian, observed that Biblical Chronologies are uncertain due to discrepancies in the figures in Genesis and other methodological factors, accounting for hundreds of different chronologies being assigned by historians. In the case of the Fathers of the Sixth Ecumenical Council, who assigned 5509 BC. as the date of the creation of man, he writes that it was in response to the emperor's wishes to fix an era or convenient starting point for historical computation. Therefore, it was a decision of mere historical convenience, not respecting either faith or morals, which are what is truly of intrinsic value in the Scriptures. Having made this disclaimer, he settles on the Benedictine Chronology of 4963 BC for the purposes of his history.
According to the ninth edition of the Encyclopædia Britannica, regarding the so-called Era of the Creation of the World, Alphonse Des Vignoles asserted in the preface to his Chronologie de l’Histoire Sainte (Chronology of Sacred History, Berlin 1738), that he collected upwards of two-hundred different calculations, the shortest of which reckons only 3483 years between the creation of the world and the commencement of the vulgar era and the longest 6984. The so-called era of the creation of the world is therefore a purely conventional and arbitrary epoch, for which the very nature of the case discussion is hopeless labour.
If there is ever a revived Roman Empire, it should use a triple year count to honor the original Roman Empire, the Byzantine Empire, and the Holy Roman Empire, so that the year might be given as Etou Kosmou 7609, AUC 2853, and AD 2100.