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
There was good reason to believe that a defeat which they suffered at the hands of the far weaker Helvetic cavalry was occasioned by themselves, and that the enemy was informed by them of all occurrences in the Roman camp. The position of Caesar grew critical; it was becoming disagreeably evident, how much the Celtic patriot party could effect even with the Haedui in spite of their official alliance with Rome, and of the distinctive interests of this canton inclining it towards the Romans; what was to be the issue, if they ventured deeper and deeper into a country full of excitement, and if they removed daily farther from their means of communication?
The armies were just marching past Bibracte (Autun), the capital of the Haedui, at a moderate distance; Caesar resolved to seize this important place by force before he continued his march into the interior; and it is very possible, that he intended to desist altogether from farther pursuit and to establish himself in Bibracte. But when he ceased from the pursuit and turned against Bibracte, the Helvetii thought that the Romans were making preparations for flight, and now attacked in their turn.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=64