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
Tigranes and Mithradates
The great-king was deeply humbled. He was of a feeble character, arrogant in prosperity, faint-hearted in adversity. Probably an agreement would have been come to between him and Lucullus-- an agreement which there was every reason that the great-king should purchase by considerable sacrifices, and the Roman general should grant under tolerable conditions--had not the old Mithradates been in existence. The latter had taken no part in the conflicts around Tigranocerta. Liberated after twenty months' captivity about the middle of 684 in consequence of the variance that had occurred between the great-king and the Romans, he had been despatched with 10,000 Armenian cavalry to his former kingdom, to threaten the communications of the enemy.
Recalled even before he could accomplish anything there, when the great-king summoned his whole force to relieve the capital which he had built, Mithradates was met on his arrival before Tigranocerta by the multitudes just fleeing from the field of battle. To every one, from the great-king down to the common soldier, all seemed lost. But if Tigranes should now make peace, not only would Mithradates lose the last chance of being reinstated in his kingdom, but his surrender would be beyond doubt the first condition of peace; and certainly Tigranes would not have acted otherwise towards him than Bocchus had formerly acted towards Jugurtha.
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Reference address : https://www.ellopos.net/elpenor/rome/5-02-rule-sullan-restoration.asp?pg=48