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
This view accords with the fact that the individual names of the months--which can only have originated after the month was viewed as part of a solar year--particularly those of March and of May, were similar among the different branches of the Italian stock, while there was no similarity between the Italian names and the Greek.
It is not improbable therefore that the problem of laying down a practical calendar which should correspond at once to the moon and the sun--a problem which may be compared in some sense to the quadrature of the circle, and the solution of which was only recognized as impossible and abandoned after the lapse of many centuries--had already employed the minds of men in Italy before the epoch at which their contact with the Greeks began; these purely national attempts to solve it, however, have passed into oblivion.
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Reference address : https://www.ellopos.net/elpenor/rome/1-14-measuring-writing.asp?pg=10