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
The Oldest Italo-Greek Calendar
What we know of the oldest calendar of Rome and of some other Latin cities--as to the Sabellian and Etruscan measurement of time we have no traditional information--is decidedly based on the oldest Greek arrangement of the year, which was intended to answer both to the phases of the moon and to the seasons of the solar year, constructed on the assumption of a lunar period of 29 1/2 days and a solar period of 12 1/2 lunar months or 368 3/4 days, and on the regular alternation of a full month or month of thirty days with a hollow month or month of twenty-nine days and of a year of twelve with a year of thirteen months, but at the same time maintained in some sort of harmony with the actual celestial phenomena by arbitrary curtailments and intercalations.
It is possible that this Greek arrangement of the year in the first instance came into use among the Latins without undergoing any alteration; but the oldest form of the Roman year which can be historically recognized varied from its model, not indeed in the cyclical result nor yet in the alternation of years of twelve with years of thirteen months, but materially in the designation and in the measuring off of the individual months.
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Reference address : https://www.ellopos.net/elpenor/rome/1-14-measuring-writing.asp?pg=11