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
For fifteen days the Roman army marched behind that of the enemy at a distance of about four miles, clinging to its rear, and hoping for an advantageous opportunity of assailing the Helvetic host under conditions favourable to victory, and destroying it. But this moment came not: unwieldy as was the march of the Helvetic caravan, the leaders knew how to guard against a surprise, and appeared to be copiously provided with supplies as well as most accurately informed by their spies of every event in the Roman camp. On the other hand the Romans began to suffer from want of necessaries, especially when the Helvetii removed from the Saone and the means of river-transport ceased.
The non-arrival of the supplies promised by the Haedui, from which this embarrassment primarily arose, excited the more suspicion, as both armies were still moving about in their territory. Moreover the considerable Roman cavalry, numbering almost 4000 horse, proved utterly untrustworthy--which doubtless admitted of explanation, for they consisted almost wholly of Celtic horsemen, especially of the mounted retainers of the Haedui, under the command of Dumnorix the well-known enemy of the Romans, and Caesar himself had taken them over still more as hostages than as soldiers.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=63