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 March in the Desert
Far and wide not an enemy showed himself; only hunger and thirst, and the endless sandy desert, seemed to keep watch at the gates of the east. At length, after many days of toilsome marching, not far from the first river which the Roman army had to cross, the Balissus (Belik), the first horsemen of the enemy were descried. Abgarus with his Arabs was sent out to reconnoitre; the Parthian squadrons retired up to and over the river and vanished in the distance, pursued by Abgarus and his followers. With impatience the Romans waited for his return and for more exact information. The general hoped here at length to come upon the constantly retreating foe; his young and brave son Publius, who had fought with the greatest distinction in Gaul under Caesar,(5) and had been sent by the latter at the head of a Celtic squadron of horse to take part in the Parthian war, was inflamed with a vehement desire for the fight.
5. Cf. V. VII. Repulse of the Helvetii, V. VII. Expeditions against the Maritime Cantons
When no tidings came, they resolved to advance at a venture; the signal for starting was given, the Balissus was crossed, the army after a brief insufficient rest at noon was led on without delay at a rapid pace. Then suddenly the kettledrums of the Parthians sounded all around; on every side their silken gold-embroidered banners were seen waving, and their iron helmets and coats of mail glittering in the blaze of the hot noonday sun; and by the side of the vizier stood prince Abgarus with his Bedouins.
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Reference address : https://www.ellopos.net/elpenor/rome/5-09-crassus-joint-rulers.asp?pg=6