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
Battle at Bibracte
Caesar desired nothing better. The two armies posted themselves on two parallel chains of hills; the Celts began the engagement, broke up the Roman cavalry which had advanced into the plain, and rushed on against the Roman legions posted on the slope of the hill, but were there obliged to give way before Caesar's veterans. When the Romans thereupon, following up their advantage, descended in their turn to the plain, the Celts again advanced against them, and a reserved Celtic corps took them at the same time in flank.
The reserve of the Roman attacking column was pushed forward against the latter; it forced it away from the main body towards the baggage and the barricade of waggons, where it was destroyed. The bulk of the Helvetic host was at length brought to give way, and compelled to beat a retreat in an easterly direction--the opposite of that towards which their expedition led them. This day had frustrated the scheme of the Helvetii to establish for themselves new settlements on the Atlantic Ocean, and handed them over to the pleasure of the victor; but it had been a hot day also for the conquerors.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=65