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
This, their first great migration, flowed past the Alps, and it was from the lands to the westward that they first began those movements of smaller masses in the opposite direction--movements which carried them over the Alps and the Haemus and even over the Bosporus, and by means of which they became and for many centuries continued to be the terror of the whole civilized nations of antiquity, till the victories of Caesar and the frontier defence organized by Augustus for ever broke their power.
The native legend of their migrations, which has been preserved to us mainly by Livy, relates the story of these later retrograde movements as follows.(8)
8. The legend is related by Livy, v. 34, and Justin, xxiv. 4, and Caesar also has had it in view (B. G. vi. 24). But the association of the migration of Bellovesus with the founding of Massilia, by which the former is chronologically fixed down to the middle of the second century of Rome, undoubtedly belongs not to the native legend, which of course did not specify dates, but to later chronologizing research; and it deserves no credit. Isolated incursions and immigrations may have taken place at a very early period; but the great overflowing of northern Italy by the Celts cannot be placed before the age of the decay of the Etruscan power, that is, not before the second half of the third century of the city.
In like manner, after the judicious investigations of Wickham and Cramer, we cannot doubt that the line of march of Bellovesus, like that of Hannibal, lay not over the Cottian Alps (Mont Genevre) and through the territory of the Taurini, but over the Graian Alps (the Little St. Bernard) and through the territory of the Salassi. The name of the mountain is given by Livy doubtless not on the authority of the legend, but on his own conjecture.
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Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=14