Nabonassar (Babylonian Nabu-Nasir)in Wikipedia
Nabonassar (also Nabonasser, Nabu-nasir, Nebo-adon-Assur or Nabo-n-assar) founded a kingdom in Babylon in 747 BC. This is now considered as the start of the Neo-Babylonian Dynasty. At the time the Assyrian Empire was in disarray through civil war and the ascendancy of other kingdoms such as Urartu. An army commander involved in the civil war, who adopted the name Tiglath-pileser III with his accession, won control of Assyria the following year 746 BC. Shortly thereafter he retook Babylon under the suzerainty of Assyria, and Nabonassar continued to rule as a vassal king for 14 years, until 734 BC.
The first of a series of tablets collectively called the Babylonian Chronicle record events beginning in the reign of Nabonassar.
The Greek astronomer Claudius Ptolemaeus started an era, i.e. a start point for chronological calculations, in the first year of his reign, on New Year's Day in the Egyptian calendar: Wednesday 26 February 747 BC in the proleptic Julian calendar. On this day the Nabonassar era (AN - Anno Nabonassari) began. The starting was used by Ptolemy because it was the earliest reign that included an astronomical observation he used, and was used later astronomers, but not by the Babylonians themselves.