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 Cimbri, Teutones, and Helvetii Unite - Expedition to Italy Resolved on - Teutones in the Province of Gaul
At length, apparently in the course of 651, the wave of the Cimbri, after having broken itself in Spain on the brave resistance of the native tribes and especially of the Celtiberians, flowed back again over the Pyrenees and thence, as it appears, passed along the shore of the Atlantic Ocean, where everything from the Pyrenees to the Seine submitted to the terrible invaders.
There, on the confines of the brave confederacy of the Belgae, they first encountered serious resistance; but there also, while they were in the territory of the Vellocassi (near Rouen), considerable reinforcements reached them. Not only three cantons of the Helvetii, including the Tigorini and Tougeni who had formerly fought against the Romans at the Garonne, associated themselves, apparently about this period, with the Cimbri, but these were also joined by the kindred Teutones under their king Teutobod, who had been driven by events which tradition has not recorded from their home on the Baltic sea to appear now on the Seine.(24)
24. The view here presented rests in the main on the comparatively trustworthy account in the Epitome of Livy (where we should read -reversi in Gallium in Vellocassis se Teutonis coniunxerunt) and in Obsequens; to the disregard of authorities of lesser weight, which make the Teutones appear by the side of the Cimbri at an earlier date, some of them, such as Appian, Celt. 13, even as early as the battle of Noreia. With these we connect the notices in Caesar (B. G. i. 33; ii. 4, 29); as the invasion of the Roman province and of Italy by the Cimbri can only mean the expedition of 652.
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Reference address : https://www.ellopos.net/elpenor/rome/4-05-peoples-north.asp?pg=43