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
Pompeius in Gaul
First of all the new general found employment in Gaul, where no formal insurrection had broken out, but serious disturbances of the peace had occurred at several places; in consequence of which Pompeius deprived the cantons of the Volcae-Arecomici and the Helvii of their independence, and placed them under Massilia. He also laid out a new road over the Cottian Alps (Mont Genevre,(21)), and so established a shorter communication between the valley of the Po and Gaul.
21. Cf. III. IV. Hannibal's Passage of the Alps
Amidst this work the best season of the year passed away; it was not till late in autumn that Pompeius crossed the Pyrenees.
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Reference address : https://www.ellopos.net/elpenor/rome/5-01-lepidus-sertorius.asp?pg=41