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
Acquisitions of Territory by Mithradates - Colchis - Northern Shores of the Black Sea
He appears more actively employed--likewise quite in the Oriental style--in enlarging on all sides his kingdom, which was even then not small, though its compass is probably over-stated at 2300 miles; we find his armies, his fleets, and his envoys busy along the Black Sea as well as towards Armenia and towards Asia Minor. But nowhere did so free and ample an arena present itself to him as on the eastern and northern shores of the Black Sea, the state of which at that time we must not omit to glance at, however difficult or in fact impossible it is to give a really distinct idea of it.
On the eastern coast of the Black Sea--which, previously almost unknown, was first opened up to more general knowledge by Mithradates--the region of Colchis on the Phasis (Mingrelia and Imeretia) with the important commercial town of Dioscurias was wrested from the native princes and converted into a satrapy of Pontus. Of still greater moment were his enterprises in the northern regions.(5)
5. They are here grouped together, because, though they were in part doubtless not executed till between the first and the second war with Rome, they to some extent preceded even the first (Memn. 30; Justin, xxxviii. 7 ap. fin.; App. Mithr. 13; Eutrop. v. 5) and a narrative in chronological order is in this case absolutely impracticable. Even the recently found decree of Chersonesus (p. 17) has given no information in this respect According to it Diophantus was twice sent against the Taurian Scythians; but that the second insurrection of these is connected with the decree of the Roman senate in favour of the Scythian princes (p. 21) is not clear from the document, and is not even probable.
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Reference address : https://www.ellopos.net/elpenor/rome/4-08-east-king-mithradates.asp?pg=14