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
Exact Sciences-- Regulation of the Calendar
That the Romans at no time distinguished themselves in the mathematical and mechanical sciences is well known, and is attested, in reference to the present epoch, by almost the only fact which can be adduced under this head with certainty--the regulation of the calendar attempted by the decemvirs. They wished to substitute for the previous calendar based on the old and very imperfect -trieteris-(29) the contemporary Attic calendar of the -octaeteris-, which retained the lunar month of 29 1/2 days but assumed the solar year at 365 1/4 days instead of 368 3/4, and therefore, without making any alteration in the length of the common year of 354 days, intercalated, not as formerly 59 days every 4 years, but 90 days every 8 years.
29. Cf. I. XIV. The Oldest Italo-Greek Calendar
With the same view the improvers of the Roman calendar intended--while otherwise retaining the current calendar--in the two inter-calary years of the four years' cycle to shorten not the intercalary months, but the two Februaries by 7 days each, and consequently to fix that month in the intercalary years at 22 and 21 days respectively instead of 29 and 28.
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=33