Heidegger's Bible Handbook: Esther: Chronology
4. Chronology. Ahasuerus, husband of Esther, appears to have been Darius, son of Hystaspes.
It is not clearly ascertained at what time the history of Esther happened. Yet it depends upon this, who אֲחַשְׁוֵרוֹשׁ/Ahasuerus, the husband of Esther, was, whom some indeed think to be Darius the Persian; others, Darius the Mede; others, Astyages; others, Cambyses; others, Xerxes; others, Artaxerxes Longimanus: but we with certain of the most learned men conclude that he was Darius, the son of Hystaspes, who was one of breakers of the Magi. For, what is said of the extent of the Kingdom of Ahasuerus, Esther 1:1, the same is said in an almost equivalent number of words concerning Darius of Hystaspes in 1 Esdras 3:1, 2 (an apocryphal book indeed, but ancient). To this Darius also is attributed the subjugation of the Indians by Herodotus, Histories, book I, as Ahasuerus is plainly said to have ruled from India to Ethiopia. The age of Mordecai also agrees, Esther 2:6, who otherwise ought to be thought to have reached more than a hundred years of age. Finally, the name also squares. For, with Valerius Maximus as a witness, Factorum et Dictorum Memorabilium, book IX, section 2, this Darius, at first called Ochus, that is, אחש, with the Royal addition, ורוש, afterwards added, was called אֲחַשְׁוֵרוֹשׁ/ Ahasuerus. But if that conjecture does not fall very far from the truth, it must be the case that this history happened in the twenty-first year after the restoration from Babylonian captivity. Moreover, the same history was conducted in the interval of about twenty years. For, in the third year of Ahasuerus, his feast is held (Esther 1:3); in the seventh year, Esther is made the wife of the King (Esther 2:16); in the twelfth year, Haman conspires against the Jews and is hanged (Esther 3:7; 7:10).
 1 Esdras 3:1, 2: “Now when Darius reigned, he made a great feast unto all his subjects, and unto all his household, and unto all the princes of Media and Persia, and to all the governors and captains and lieutenants that were under him, from India unto Ethiopia, of an hundred twenty and seven provinces.”  Valerius Maximus was a first century Roman collector of antiquities.