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 Roman year began with the beginning of spring; the first month in it and the only one which bears the name of a god, was named from Mars (-Martius-), the three following from sprouting (-aprilis-) growing (-maius-), and thriving (-iunius-), the fifth onward to the tenth from their ordinal numbers (-quinctilis-, -sextilis-, -september-, -october-, -november-, -december), the eleventh from commencing (-ianuarius-),(8) with reference presumably to the renewal of agricultural operations that followed midwinter and the season of rest, the twelfth, and in an ordinary year the last, from cleansing (-februarius-).
8. Cf. I. XII. Nature of the Roman Gods
To this series recurring in regular succession there was added in the intercalary year a nameless "labour-month" (-mercedonius-) at the close of the year, viz. after February. And, as the Roman calendar was independent as respected the names of the months which were probably taken from the old national ones, it was also independent as regarded their duration. Instead of the four years of the Greek cycle, each composed of six months of 30 and six of 29 days and an intercalary month inserted every second year alternately of 29 and 30 days (354 + 384 + 354 + 383 = 1475 days), the Roman calendar substituted four years, each containing four months--the first, third, fifth, and eighth--of 31 days and seven of 29 days, with a February of 28 days during three years and of 29 in the fourth, and an intercalary month of 27 days inserted every second year (355 + 383 + 355 + 382 = 1475 days).
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Reference address : https://www.ellopos.net/elpenor/rome/1-14-measuring-writing.asp?pg=12