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
In spite of the differences in language and manners which naturally were to be found within this wide territory, a close mutual intercourse, an innate sense of fellowship, seems to have knit together the tribes from the Rhone and Garonne to the Rhine and the Thames; whereas, although these doubtless were in a certain measure locally connected with the Celts in Spain and in the modern Austria, the mighty mountain barriers of the Pyrenees and the Alps on the one hand, and the encroachments of the Romans and the Germans which also operated here on the other, interrupted the intercourse and the intrinsic connection of the cognate peoples far otherwise than the narrow arm of the sea interrupted the relations of the continental and the British Celts.
Unhappily we are not permitted to trace stage by stage the history of the internal development of this remarkable people in these its chief seats; we must be content with presenting at least some outline of its historical culture and political condition, as it here meets us in the time of Caesar.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=14