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
But want of mathematical precision and theological scruples, especially in reference to the annual festival of Terminus which fell within those very days in February, disarranged the intended reform, so that the Februaries of the intercalary years came to be of 24 and 23 days, and thus the new Roman solar year in reality ran to 366 1/4 days. Some remedy for the practical evils resulting from this was found in the practice by which, setting aside the reckoning by the months or ten months of the calendar (30) as now no longer applicable from the inequality in the length of the months, wherever more accurate specifications were required, they accustomed themselves to reckon by terms of ten months of a solar year of 365 days or by the so-called ten-month year of 304 days.
30. Cf. I. XIV. The Oldest Italo-Greek Calendar
Over and above this, there came early into use in Italy, especially for agricultural purposes, the farmers' calendar based on the Egyptian solar year of 365 1/4 days by Eudoxus (who flourished 386).
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=34