The Seleucid era or Anno Graecorum, 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 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. 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 but reckoned the new year to be in the autumn. 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 westSyrian 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: The Seleucid era was used as late as the 6th century AD, for instance in the in Syria, dated the 24th of Gorpiaios, 823, and in the writings of John of Ephesus. Syriac chroniclers continued to use it up to Michael the Syrian in the 12th century AD / 15th century AG. It has been found on Nestorian Christian tombstones from Central Asia well into the 14th centuryAD. The Seleucid era counting, or "era of contracts", 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 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.