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.
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.
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 was used as late as the 6th century... Read More