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
Attempt at Relief - Conflicts before Alesia
At the last hour there appeared behind Caesar's lines the interminable array of the Celto-Belgic relieving array, said to amount to 250,000 infantry and 8000 cavalry, from the Channel to the Cevennes the insurgent cantons had strained every nerve to rescue the flower of their patriots and the general of their choice--the Bellovaci alone had answered that they were doubtless disposed to fight against the Romans, but not beyond their own bounds.
The first assault, which the besieged of Alesia and the relieving troops without made on the Roman double line, was repulsed; but, when after a day's rest it was repeated, the Celts succeeded--at a spot where the line of circumvallation ran over the slope of a hill and could be assailed from the height above-- in filling up the trenches and hurling the defenders down from the rampart. Then Labienus, sent thither by Caesar, collected the nearest cohorts and threw himself with four legions on the foe. Under the eyes of the general, who himself appeared at the most dangerous moment, the assailants were driven back in a desperate hand-to-hand conflict, and the squadrons of cavalry that came with Caesar taking the fugitives in rear completed the defeat.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=128