The era of Alexander (Seleucid era)

The era of Alexander is one of the well-known eras that al-Biruni treats in Book 2 of al-Qanun al-Mas'udi; it was in use in Syria and Iraq and in the zijes. ¹

Al-Biruni says, as the editor quotes, that most people suppose the era of Alexander counted from the beginning of his reign, like the era of Yazdgird from his first year, and that the authors of the reasons of the zijes said the first year of Alexander's reign began on a Monday; when they found Ptolemy dating some of his observations from Alexander's death, which fell earlier than what they took for his accession, they were perplexed. ¹

Al-Biruni explains that Philip ruled Macedonia for twenty-seven years, that Alexander was born to him of Olympias in the eighth year of his reign, and that Alexander reigned after his father twelve years and seven months, six of them until he killed Darius and the rest in conquering the East. ² When he died at Babylon his realm was divided in three: Macedonia went to his brother Philip Arrhidaeus, from whom Theon's canon dates; Egypt and the West to the Ptolemies, the first being Ptolemy son of Lagus; and Syria and Iraq to Antiochus, builder of Antioch. ² Seleucus shared rule with Antiochus until he ruled alone at the end of the twelfth year of the son of Lagus; from then the Greeks began to date, and the era became known by Alexander's name though it starts in the thirteenth year after his death. ²

The editor cites the passage to show that al-Biruni did not list eras thoughtlessly but examined their origins and the reasons for the historians' disagreements. ²