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 constant apprehensions on the part of the Celts as to such a Roman invasion were therefore sufficiently justified; but the Romans were in no haste. So the Celts on their part began the war, either because the Roman assignations of land on the east coast (522), although not a measure immediately directed against them, made them apprehensive of danger; or because they perceived that a war with Rome for the possession of Lombardy was inevitable; or, as is perhaps most probable, because their Celtic impatience was once more weary of inaction and preferred to arm for a new warlike expedition.
With the exception of the Cenomani, who acted with the Veneti and declared for the Romans, all the Italian Celts concurred in the war, and they were joined by the Celts of the upper valley of the Rhone, or rather by a number of adventurers belonging to them, under the leaders Concolitanus and Aneroestus.(15)
15. These, whom Polybius designates as the "Celts in the Alps and on the Rhone, who on account of their character as military adventurers are called Gaesatae (free lances)," are in the Capitoline Fasti named -Germani-. It is possible that the contemporary annalists may have here mentioned Celts alone, and that it was the historical speculation of the age of Caesar and Augustus that first induced the redactors of these Fasti to treat them as "Germans." If, on the other hand, the mention of the Germans in the Fasti was based on contemporary records --in which case this is the earliest mention of the name--we shall here have to think not of the Germanic races who were afterwards so called, but of a Celtic horde.
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Reference address : https://www.ellopos.net/elpenor/rome/3-03-extension-italy.asp?pg=30