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 Haedui Waver
The Haedui, who had hitherto continued vacillating, now made preparations in earnest to join the patriotic party; the body of men, whom Caesar had ordered to Gergovia, had on the march been induced by its officers to declare for the insurgents; at the same time they had begun in the canton itself to plunder and kill the Romans settled there. Caesar, who had gone with two-thirds of the blockading army to meet that corps of the Haedui which was being brought up to Gergovia, had by his sudden appearance recalled it to nominal obedience; but it was more than ever a hollow and fragile relation, the continuance of which had been almost too dearly purchased by the great peril of the two legions left behind in front of Gergovia.
For Vercingetorix, rapidly and resolutely availing himself of Caesar's departure, had during his absence made an attack on them, which had wellnigh ended in their being overpowered, and the Roman camp being taken by storm. Caesar's unrivalled celerity alone averted a second catastrophe like that of Aduatuca. Though the Haedui made once more fair promises, it might be foreseen that, if the blockade should still be prolonged without result, they would openly range themselves on the side of the insurgents and would thereby compel Caesar to raise it; for their accession would interrupt the communication between him and Labienus, and expose the latter especially in his isolation to the greatest peril. Caesar was resolved not to let matters come to this pass, but, however painful and even dangerous it was to retire from Gergovia without having accomplished his object, nevertheless, if it must be done, rather to set out immediately and by marching into the canton of the Haedui to prevent at any cost their formal desertion.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=120