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
Second Pontic Army Sent to Greece - Battle of Orchomenus
In the spring of 669 there was again fresh work in Europe. Mithradates, who continued his preparations indefatigably in Asia Minor, had sent an army not much less than that which had been extirpated at Chaeronea, under Dorylaus to Euboea; thence it had, after a junction with the remains of the army of Archelaus, passed over the Euripus to Boeotia. The Pontic king, who judged of what his army could do by the standard of victories over the Bithynian and Cappadocian militia, did not understand the unfavourable turn which things had taken in Europe; the circles of the courtiers were already whispering as to the treason of Archelaus; peremptory orders were issued to fight a second battle at once with the new army, and not to fail on this occasion to annihilate the Romans.
The master's will was carried out, if not in conquering, at least in fighting. The Romans and Asiatics met once more in the plain of the Cephissus, near Orchomenus. The numerous and excellent cavalry of the latter flung itself impetuously on the Roman infantry, which began to waver and give way: the danger was so urgent, that Sulla seized a standard and advancing with his adjutants and orderlies against the enemy called out with a loud voice to the soldiers that, if they should be asked at home where they had abandoned their general, they might reply--at Orchomenus.
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Reference address : http://www.ellopos.net/elpenor/rome/4-08-east-king-mithradates.asp?pg=58