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
But on the right bank here the rocks and precipices come so close to the river that there remained only a narrow path which could easily be blocked up, and the Sequani, to whom this bank belonged, could with ease intercept the route of the Helvetii. They preferred therefore to pass over, above the point where the Rhone breaks through, to the left Allobrogian bank, with the view of regaining the right bank further down the stream where the Rhone enters the plain, and then marching on towards the level west of Gaul; there the fertile canton of the Santones (Saintonge, the valley of the Charente) on the Atlantic Ocean was selected by the wanderers for their new abode.
This march led, where it touched the left bank of the Rhone, through Roman territory; and Caesar, otherwise not disposed to acquiesce in the establishment of the Helvetii in western Gaul, was firmly resolved not to permit their passage. But of his four legions three were stationed far off at Aquileia; although he called out in haste the militia of the Transalpine province, it seemed scarcely possible with so small a force to hinder the innumerable Celtic host from crossing the Rhone, between its exit from the Leman lake at Geneva and the point of its breaking through the mountains, over a distance of more than fourteen miles.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=59