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
That the Umbrians were after obstinate struggles driven back from the north and west into the narrow mountainous country between the two arms of the Apennines which they subsequently held, is clearly indicated by the very fact of their geographical position, just as the position of the inhabitants of the Grisons and that of the Basques at the present day indicates the similar fate that has befallen them. Tradition also has to report that the Tuscans wrested from the Umbrians three hundred towns; and, what is of more importance as evidence, in the national prayers of the Umbrian Iguvini, which we still possess, along with other stocks the Tuscans especially are cursed as public foes.
In consequence, as may be presumed, of this pressure exerted upon them from the north, the Umbrians advanced towards the south, keeping in general upon the heights, because they found the plains already occupied by Latin stocks, but beyond doubt frequently making inroads and encroachments on the territory of the kindred race, and intermingling with them the more readily, that the distinction in language and habits could not have been at all so marked then as we find it afterwards. To the class of such inroads belongs the tradition of the irruption of the Reatini and Sabines into Latium and their conflicts with the Romans; similar phenomena were probably repeated all along the west coast.
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Reference address : https://www.ellopos.net/elpenor/rome/1-08-umbro-sabellian-samnites.asp?pg=3