The Seleucid era or Anno Graecorum (literally "year of the Greeks" or "Greek year"), sometimes denoted "AG", was a system of numbering years in use by the Seleucid Empire and other countries among the ancient Hellenistic civilizations. It is sometimes referred to as "the dominion of the Seleucidæ," or the Year of Alexander. The era dates from Seleucus I Nicator's re-conquest of Babylon in 312/11 BC after his exile in Ptolemaic Egypt, considered by Seleucus and his court to mark the founding of the Seleucid Empire. According to Jewish tradition, it was during the sixth year of Alexander the Great's reign (lege: possibly Alexander the Great's infant son, Alexander IV of Macedon) that they began to make use of this counting. The introduction of the new era is mentioned in one of the Babylonian Chronicles, the Chronicle of the Diadochi.
Two different uses were made of the Seleucid years:
The natives of the empire used the Babylonian calendar, in which the new year falls on 1 Nisanu (3 April in 311 BC), so in this system year 1 of the Seleucid era corresponds roughly to April 311 BC to March 310 BC. This included the Jews, who call it the Era of Contracts Hebrew מניין שטרות, minyan shtarot). It is used in the Jewish historical book, now "deuterocanonical", 1 Maccabees, in 6:20, 7:1, 9:3, 10:1, etc.
The Macedonian court adopted the Babylonian calendar (substituting the Macedonian month names) but reckoned the new year to be in the autumn (the exact date is unknown). In this system year 1 of the Seleucid era corresponds to the period from autumn 312 BC to summer 311 BC. By the 7th century AD / 10th AG, the west Syrian Christians settled on 1 October-to-30 September. Jews, however, reckon the start of each new Seleucid year with the lunar month Tishri.
These differences in the beginning of the year mean that dates may differ by one. Bickerman gives this example:
For instance, the restoration of the temple of Jerusalem by Judas Maccabaeus, approximately 15 December 164 BC, fell in the year 148 of the Seleucid Era according to Jewish (and Babylonian) calculation, but in the year 149 for the court.
The Seleucid era counting, or "era of contracts" (minyan sheṭarot), was used by Yemenite Jews in their legal deeds and contracts until modern times, a practice derived from an ancient Jewish teaching in the Talmud, requiring all Diaspora Jews to uphold its practice. For this reason, the Seleucid era counting is mentioned in the Book of Maccabees (I Macc. i. 11) and in the writings of the historian, Josephus. The Seleucid era counting fell into disuse among most Jewish communities, following Rabbi David ben Zimra's cancellation of the practice when he served as Chief Rabbi of Egypt.
^Denis C. Feeney, Caesar's Calendar, University of California Press, Berkeley 2007, p. 139.
^Babylonian Talmud (Avodah Zarah 10a), Rabbeinu Hananel's Commentary; RASHI's commentary on Babylonian Talmud (Avodah Zarah 9a); Sefer Hakabbalah of Rabbi Avraham ben David (Ravad); Midrash David on Mishnah Tractate Avot (Ethics of the Fathers, 1:6)
Kosmin, Paul J. (7 May 2019). "A revolution in time". aeon. Retrieved 12 January 2020. Once local and irregular, time-keeping became universal and linear in 311 BCE. History would never be the same again.