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
The Romans and Mithradates - Intervention of the Senate
That the Roman senate asserted its general policy--of keeping down
the states more or less dependent on it--also in dealing with that
of Pontus, is shown by its attitude on occasion of the succession to
the throne after the sudden death of Mithradates V. From the boy in
minority who followed him there was taken away Great Phrygia, which
had been conferred on his father for his taking part in the war
against Aristonicus or rather for his good money,(9) and this region
was added to the territory immediately subject to Rome.(10)
10. A decree of the senate of the year 638 recently found in the
village Aresti to the south of Synnada (Viereck, -Sermo Graecus quo
senatus Romanus usus sit-, p. 51) confirms all the regulations made
by the king up to his death and thus shows that Great Phrygia after
the death of the father was not merely taken from the son, as Appian
also states, but was thereby brought directly under Roman allegiance.