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
Caesar before Gergovia - Fruitless Blockade
The Roman main army in its turn advanced along the Allier down into the canton of the Arverni. Vercingetorix attempted to prevent it from crossing to the left bank of the Allier, but Caesar overreached him and after some days stood before the Arvernian capital Gergovia.(48)
48. This place has been sought on a rising ground which is still named Gergoie, a league to the south of the Arvernian capital Nemetum, the modern Clermont; and both the remains of rude fortress-walls brought to light in excavations there, and the tradition of the name which is traced in documents up to the tenth century, leave no room for doubt as to the correctness of this determination of the locality. Moreover it accords, as with the other statements of Caesar, so especially with the fact that he pretty clearly indicates Gergovia as the chief place of the Arverni (vii. 4). We shall have accordingly to assume, that the Arvernians after their defeat were compelled to transfer their settlement from Gergovia to the neighbouring less strong Nemetum.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=118