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
The Helvetii Move towards Gaul
On the other hand, the party in Gaul hostile to the Romans, which hoped to obtain a powerful reinforcement in the Helvetii, more especially the Haeduan Dumnorix brother of Divitiacus, and at the head of the national party in his canton as the latter wasat the head of the Romans, procured for them a passage through the passes of the Jura and the territory of the Sequani.
The Romans had no legal title to forbid this; but other and higher interestswereat stake for them in the Helvetic expedition than the question of the formal integrity of the Roman territory-- interests which could only be guarded, if Caesar, instead of confining himself, as all the governors of the senate and even Marius(34) had done, to the modest task of watching the frontier, should cross what had hitherto been the frontier at the head of a considerable army.
34. Cf. IV. V. The Cimbri, Teutones, and Helvetii Unite
Caesar was general not of the senate, but of the state; he showed no hesitation. He had immediately proceeded from Genava in person to Italy, and with characteristic speed brought up the three legions cantoned there as well as two newly-formed legions of recruits.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=61