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
Men recalled the never wholly forgotten Celtic inroads of the fourth century, the day on the Allia and the burning of Rome: with the double force at once of the oldest remembrance and of the freshest alarm the terror of the Gauls came upon Italy; through all the west people seemed to be aware that the Roman empire was beginning to totter. As after the battle of Cannae, the period of mourning was shortened by decree of the senate.(21)
21. To this, beyond doubt, the fragment of Diodorus (Vat. p. 122) relates.
The new enlistments brought out the most painful scarcity of men. All Italians capable of bearing arms had to swear that they would not leave Italy; the captains of the vessels lying in the Italian ports were instructed not to take on board any man fit for service. It is impossible to tell what might have happened, had the Cimbri immediately after their double victory advanced through the gates of the Alps into Italy. But they first overran the territory of the Arverni, who with difficulty defended themselves in their fortresses against the enemy; and soon, weary of sieges, set out from thence, not to Italy, but westward to the Pyrenees.
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Reference address : https://www.ellopos.net/elpenor/rome/4-05-peoples-north.asp?pg=36