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 Italian Calendar before the Period of Greek Influence in Italy
In like manner the Roman calendar--and probably that of the Italians generally--began with an independent development of its own, but subsequently came under the influence of the Greeks. In the division of time the returns of sunrise and sunset, and of the new and full moon, most directly arrest the attention of man; and accordingly the day and the month, determined not by cyclic calculation but by direct observation, were long the exclusive measures of time.
Down to a late age sunrise and sunset were proclaimed in the Roman market-place by the public crier, and in like manner it may be presumed that in earlier times, at each of the four phases of the moon, the number of days that would elapse from that phase until the next was proclaimed by the priests. The mode of reckoning therefore in Latium--and the like mode, it may be presumed, was in use not merely among the Sabellians, but also among the Etruscans--was by days, which, as already mentioned, were counted not forward from the phase that had last occurred, but backward from that which was next expected; by lunar weeks, which varied in length between 7 and 8 days, the average length being 7 3/8; and by lunar months which in like manner were sometimes of 29, sometimes of 30 days, the average duration of the synodical month being 29 days 12 hours 44 minutes.
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Reference address : https://www.ellopos.net/elpenor/rome/1-14-measuring-writing.asp?pg=8