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 real genesis of the Latin and doubtless also of the other Italian languages was anterior to this period, and that even at its commencement the Latin language was substantially an accomplished fact, is evident from the fragments of the Twelve Tables, which, however, have been largely modernized by their semi-oral tradition.
They contain doubtless a number of antiquated words and harsh combinations, particularly in consequence of omitting the indefinite subject; but their meaning by no means presents, like that of the Arval chant, any real difficulty, and they exhibit far more agreement with the language of Cato than with that of the ancient litanies.
If the Romans at the beginning of the seventh century had difficulty in understanding documents of the fifth, the difficulty doubtless proceeded merely from the fact that there existed at that time in Rome no real, least of all any documentary, research.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=27