Xerxes’ rule is customarily counted from 486 B.C.E., when Darius, his father, died.

Darius I (called Darius Hystaspis or Darius the Great) evidently engineered or instigated the slaying of the one occupying the Persian throne and gained the throne for himself. During his rule the temple work at Jerusalem was renewed with royal approval, and the temple was completed during his sixth year of rule (early in 515 B.C.E.). (Ezr 6:1-15) Darius’ reign was one of imperial expansion. He extended Persian dominion as far E as India and as far W as Thrace and Macedonia.

At least by this time the Persian rulers had fulfilled the prophetic symbolisms of Daniel 7:5 and 8:4, where, under the symbols of a bear and also a ram, the Medo-Persian Empire is represented as seizing territories in three principal directions: to the N, the W, and the S. In a campaign against Greece, however, Darius’ forces suffered defeat at Marathon in 490 B.C.E. Darius died in 486 B.C.E.​—See DARIUS No. 2.

If Darius died in 486 B.C.E. and Xerxes died in 475 B.C.E., how could it be explained that some ancient documents allot to Xerxes a reign of 21 years? It is well known that a king and his son might rule together in a double kingship, or coregency. If this was the case with Darius and Xerxes, historians could count the years of Xerxes’ reign either from the start of a coregency with his father or from his father’s death. If Xerxes ruled 10 years with his father and 11 years by himself, some sources could attribute to him 21 years of rulership, while others might give him 11 years.

There is solid evidence for a coregency of Xerxes with his father Darius. The Greek historian Herodotus (VII, 3) says: “Darius judged his [Xerxes’] plea [for kingship] to be just and declared him king. But to my thinking Xerxes would have been made king even without this advice.” This indicates that Xerxes was made king during the reign of his father Darius.

Evidence from Persian sources. A coregency of Xerxes with Darius can be seen especially from Persian bas-reliefs that have come to light. In Persepolis several bas-reliefs have been found that represent Xerxes standing behind his father’s throne, dressed in clothing identical to his father’s and with his head on the same level. This is unusual, since ordinarily the king’s head would be higher than all others. In A New Inscription of Xerxes From Persepolis (by Ernst E. Herzfeld, 1932) it is noted that both inscriptions and buildings found in Persepolis imply a coregency of Xerxes with his father Darius. On page 8 of his work Herzfeld wrote: “The peculiar tenor of Xerxes’ inscriptions at Persepolis, most of which do not distinguish between his own activity and that of his father, and the relation, just as peculiar, of their buildings, which it is impossible to allocate to either Darius or Xerxes individually, have always implied a kind of coregency of Xerxes. Moreover, two sculptures at Persepolis illustrate that relation.” With reference to one of these sculptures, Herzfeld pointed out: “Darius is represented, wearing all the royal attributes, enthroned on a high couch-platform supported by representatives of the various nations of his empire. Behind him in the relief, that is, in reality at his right, stands Xerxes with the same royal attributes, his left hand resting on the high back of the throne. That is a gesture that speaks clearly of more than mere successorship; it means coregency.”

As to a date for reliefs depicting Darius and Xerxes in that way, in Achaemenid Sculpture (Istanbul, 1974, p. 53), Ann Farkas states that “the reliefs might have been installed in the Treasury sometime during the building of the first addition, 494/493–492/491 B.C.; this certainly would have been the most convenient time to move such unwieldy pieces of stone. But whatever their date of removal to the Treasury, the sculptures were perhaps carved in the 490’s.”

Evidence from Babylonian sources. Evidence for Xerxes beginning a coregency with his father during the 490’s B.C.E. has been found at Babylon. Excavations there have unearthed a palace for Xerxes completed in 496 B.C.E. In this regard, A. T. Olmstead wrote in History of the Persian Empire (p. 215): “By October 23, 498, we learn that the house of the king’s son [that is, of Darius’ son, Xerxes] was in process of erection at Babylon; no doubt this is the Darius palace in the central section that we have already described. Two years later [in 496 B.C.E.], in a business document from near-by Borsippa, we have reference to the ‘new palace’ as already completed.”

Two unusual clay tablets may bear additional testimony to the coregency of Xerxes with Darius. One is a business text about hire of a building in the accession year of Xerxes. The tablet is dated in the first month of the year, Nisan. (A Catalogue of the Late Babylonian Tablets in the Bodleian Library, Oxford, by R. Campbell Thompson, London, 1927, p. 13, tablet designated A. 124) Another tablet bears the date “month of Ab(?), accession year of Xerxes.” Remarkably, this latter tablet does not attribute to Xerxes the title “king of Babylon, king of lands,” which was usual at that time.​—Neubabylonische Rechts- und Verwaltungsurkunden übersetzt und erläutert, by M. San Nicolò and A. Ungnad, Leipzig, 1934, Vol. I, part 4, p. 544, tablet No. 634, designated VAT 4397.

These two tablets are puzzling. Ordinarily a king’s accession year begins after the death of his predecessor. However, there is evidence that Xerxes’ predecessor (Darius) lived until the seventh month of his final year, whereas these two documents from the accession year of Xerxes bear dates prior to the seventh month (one has the first month, the other the fifth). Therefore these documents do not relate to an accession period of Xerxes following the death of his father but indicate an accession year during his coregency with Darius. If that accession year was in 496 B.C.E., when the palace at Babylon for Xerxes had been completed, his first year as coregent would begin the following Nisan, in 495 B.C.E., and his 21st and final year would start in 475 B.C.E. In that case, Xerxes’ reign included 10 years of rule with Darius (from 496 to 486 B.C.E.) and 11 years of kingship by himself (from 486 to 475 B.C.E.).