Claudius Ptolemy, a mathematician, geographer, historian and astrologer lived in Alexandria, Egypt during the turn of the second century AD. He was a Greek-Roman citizen linked to the great Alexandria Library. For our purposes, we are most interested in his Canon of Kings used by ancient astronomers to mark dates and times for astronomical obsevations like eclipses. His preservation of this king's list became one of the most important contributions to the study of ancient chronology.
The starting point of Ptolemy's Nabonassar era on the first day of the Egyptian month, Thoth in the year 747 BC (February 26) is established by 19 lunar eclipses Therefore, we can locate any year of any of these kings as reckoned by the Egyptian calendar year, and we can compute that date in our own BC dating system.