Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Death of Mithadates
After the country and the army had abandoned the king, the capital Panticapaeum at length opened its gates to the insurgents and delivered over to them the old king enclosed in his palace. From the high wall of his castle the latter besought his son at least to grant him life and not imbrue his hands in his father's blood; but the request came ill from the lips of a man whose own hands were stained with the blood of his mother and with the recently-shed blood of his innocent son Xiphares; and in heartless severity and inhumanity Pharnaces even outstripped his father.
Seeing therefore he had now to die, the sultan resolved at least to die as he had lived; his wives, his concubines and his daughters, including the youthful brides of the kings of Egypt and Cyprus, had all to suffer the bitterness of death and drain the poisoned cup, before he too took it, and then, when the draught did not take effect quickly enough, presented his neck for the fatal stroke to a Celtic mercenary Betuitus. So died in 691 Mithradates Eupator, in the sixty-eighth year of his life and the fifty-seventh of his reign, twenty-six years after he had for the first time taken the field against the Romans. The dead body, which king Pharnaces sent as a voucher of his merits and of his loyalty to Pompeius, was by order of the latter laid in the royal sepulchre of Sinope.
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Reference address : https://www.ellopos.net/elpenor/rome/5-04-pompeius-east.asp?pg=28