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
Lesser Armenia - Alliance with Tigranes
While thus the most important successes were gained towards the north, the king at the same time extended his dominions towards the east and the west. The Lesser Armenia was annexed by him and converted from a dependent principality into an integral part of the Pontic kingdom; but still more important was the close connection which he formed with the king of the Greater Armenia.
He not only gave his daughter Cleopatra in marriage to Tigranes, but it was mainly through his support that Tigranes shook off the yoke of the Arsacids and took their place in Asia. An agreement seems to have been made between the two to the effect that Tigranes should take in hand to occupy Syria and the interior of Asia, and Mithradates Asia Minor and the coasts of the Black Sea, under promise of mutual support; and it was beyond doubt the more active and capable Mithradates who brought about this agreement with a view to cover his rear and to secure a powerful ally.
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Reference address : https://www.ellopos.net/elpenor/rome/4-08-east-king-mithradates.asp?pg=22