Parker, R.A.; Dubberstein, Waldo H. (1956). Babylonian Chronology, 626 B.C.–A.D. 75. Providence: Brown University Press. p. 36. OCLC460027103.. There, they conclude that this same period spanned from the first year of Darius I, in 522 BCE, to the 6th-year of Alexander the Great, in 331 BCE, a time-span of 191 years.