The Greek historian Herodotus (c. 485-425 B.C.E.) stated that Cyrus was killed “after he had reigned twenty-nine years,” which would put his death during his 30th year, in 530 B.C.E. (Histories, Book I, Clio, 214)

A Babylonian astronomical clay tablet (BM 33066) confirms the date of Cyrus’ death in 530 B.C.E. Though this tablet contains some errors regarding the astronomical positions, it contains the descriptions of two lunar eclipses that the tablet says occurred in the seventh year of Cambyses II, the son and successor of Cyrus. These are identified with lunar eclipses visible at Babylon on July 16, 523 B.C.E., and on January 10, 522 B.C.E., thus pointing to the spring of 523 B.C.E. as the beginning of Cambyses’ seventh year. That would make his first regnal year 529 B.C.E. So Cyrus’ last year would have been 530 B.C.E., making 539 B.C.E. his first year of ruling Babylon.