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 capitulation was virtually concluded, when Viriathus--a man of humble origin, who formerly, when a youth, had bravely defended his flock from wild beasts and robbers and was now in more serious conflictsa dreaded guerilla chief, and who was one of the few that had accidentally escaped from the perfidious onslaught of Galba--warned his countrymen against relying on the Roman word of honour, and promised them deliverance if they would follow him. His language and his example produced a deep effect: the army entrusted him with the supreme command.
Viriathus gave orders to the mass of his men to proceed in detached parties, by different routes, to the appointed rendezvous; he himself formed the best mounted and most trustworthy into a corps of 1000 horse, with which he covered the departure of his men. The Romans, who wanted light cavalry, did not venture to disperse for the pursuit under the eyes of the enemy's horsemen. After Viriathus and his band had for two whole days held in check the entire Roman army he suddenly disappeared during the night and hastened to the general rendezvous. The Roman general followed him, but fell into an adroitly-laid ambush, in which he lost the half of his army and was himself captured and slain; with difficulty the rest of the troops escaped to the colony of Carteia on the Straits.
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Reference address : https://www.ellopos.net/elpenor/rome/4-01-gracchi.asp?pg=13