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
Repulse of the Helvetii
Just at this time the Helvetic invasion, which was closely interwoven with the German and had been in preparation for years, began. That they might not make a grant of their abandoned huts to the Germans and might render their own return impossible, the Helvetii had burnt their towns and villages; and their long trains of waggons, laden with women, children, and the best part of their moveables, arrived from all sides at the Leman lake near Genava (Geneva), where they and their comrades had fixed their rendezvous for the 28th of March(33) of this year.
33. According to the uncorrected calendar. According to the current rectification, which however here by no means rests on sufficiently trustworthy data, this day corresponds to the 16th of April of the Julian calendar.
According to their own reckoning the whole body consisted of 368,000 persons, of whom about a fourth part were able to bear arms. As the mountain chain of the Jura, stretching from the Rhine to the Rhone, almost completely closed in the Helvetic country towards the west, and its narrow defiles were as ill adapted for the passage of such a caravan as they were well adapted for defence, the leaders had resolved to go round in a southerly direction, and to open up for themselves a way to the west at the point, where the Rhone has broken through the mountain-chain between the south-western and highest part of the Jura and the Savoy mountains, near the modern Fort de l'Ecluse.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=58