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
Celtic Migrations - The Celts Assail the Etruscans in Northern Italy
Such is the description which the ancients give us of this nation. Its origin can only be conjectured. Sprung from the same cradle from which the Greek, Italian, and Germanic peoples issued,(7) the Celts doubtless like these migrated from their eastern motherland into Europe, where at a very early period they reached the western ocean and established their headquarters in what is now France, crossing to settle in the British isles on the north, and on the south passing the Pyrenees and contending with the Iberian tribes for the possession of the peninsula.
7. It has recently been maintained by expert philologists that there is a closer affinity between the Celts and Italians than there is even between the latter and the Greeks. In other words they hold that the branch of the great tree, from which the peoples of Indo-Germanic extraction in the west and south of Europe have sprung, divided itself in the first instance into Greeks and Italo-Celts, and that the latter at a considerably later period became subdivided into Italians and Celts.
This hypothesis commends itself much to acceptance in a geographical point of view, and the facts which history presents may perhaps be likewise brought into harmony with it, because what has hitherto been regarded as Graeco-Italian civilization may very well have been Graeco-Celto-Italian--in fact we know nothing of the earliest stage of Celtic culture. Linguistic investigation, however, seems not to have made as yet such progress as to warrant the insertion of its results in the primitive history of the peoples.
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Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=13