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
The Parthian State - Armenia
The monarchy of the Arsacids had to pass through a dangerous crisis about the time of the Gracchi, chiefly in consequence of the inroads of Turanian tribes. The ninth Arsacid, Mithradates II or the Great (630?-667?), had recovered for the state its position of ascendency in the interior of Asia, repulsed the Scythians, and advanced the frontier of the kingdom towards Syria and Armenia; but towards the end of his life new troubles disturbed his reign; and, while the grandees of the kingdom including his own brother Orodes rebelled against the king and at length that brother overthrew him and had put him to death, the hitherto unimportant Armenia rose into power.
This country, which since its declaration of independence(2) had been divided into the north-eastern portion or Armenia proper, the kingdom of the Artaxiads, and the south-western or Sophene, the kingdom of the Zariadrids, was for the first time united into one kingdom by the Artaxiad Tigranes (who had reigned since 660); and this doubling of his power on the one hand, and the weakness of the Parthian rule on the other, enabled the new king of all Armenia not only to free himself from dependence on the Parthians and to recover the provinces formerly ceded to them, but even to bring to Armenia the titular supremacy of Asia, as it had passed from the Achaemenids to the Seleucids and from the Seleucids to the Arsacids.
2. Cf. III. IX. Armenia
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Reference address : https://www.ellopos.net/elpenor/rome/4-08-east-king-mithradates.asp?pg=4