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
It thus appears that, while the Italian language holds an independent position by the side of the Greek, the Latin dialect within it bears a relation to the Umbro-Samnite somewhat similar to that of the Ionic to the Doric; and the differences of the Oscan and Umbrian and kindred dialects may be compared with the differences between the Dorism of Sicily and the Dorism of Sparta.
Each of these linguistic phenomena is the result and the attestation of an historical event. With perfect certainty they guide us to the conclusion, that from the common cradle of peoples and languages there issued a stock which embraced in common the ancestors of the Greeks and the Italians; that from this, at a subsequent period, the Italians branched off; and that these again divided into the western and eastern stocks, while at a still later date the eastern became subdivided into Umbrians and Oscans.
When and where these separations took place, language of course cannot tell; and scarce may adventurous thought attempt to grope its conjectural way along the course of those revolutions, the earliest of which undoubtedly took place long before that migration which brought the ancestors of the Italians across the Apennines. On the other hand the comparison of languages, when conducted with accuracy and caution, may give us an approximate idea of the degree of culture which the people had reached when these separations took place, and so furnish us with the beginnings of history, which is nothing but the development of civilization. For language, especially in the period of its formation, is the true image and organ of the degree of civilization attained; its archives preserve evidence of the great revolutions in arts and in manners, and from its records the future will not fail to draw information as to those times regarding which the voice of direct tradition is dumb.
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Reference address : https://www.ellopos.net/elpenor/rome/1-02-earliest-migrations-italy.asp?pg=10