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
Vercingetorix, however, doubtless even while he was confronting Caesar on the Allier, had caused sufficient stores to be collected in Gergovia and a fixed camp provided with strong stone ramparts to be constructed for his troops in front of the walls of the town, which was situated on the summit of a pretty steep hill; and, as he had a sufficient start, he arrived before Caesar at Gergovia and awaited the attack in the fortified camp under the wall of the fortress.
Caesar with his comparatively weak army could neither regularly besiege the place nor even sufficiently blockade it; he pitched his camp below the rising ground occupied by Vercingetorix, and was compelled to preserve an attitude as inactive as his opponent. It was almost a victory for the insurgents, that Caesar's career of advance from triumph to triumph had been suddenly checked on the Seine as on the Allier. In fact the consequences of this check for Caesar were almost equivalent to those of a defeat.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=119