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 retention moreover of the reckoning by months or--which is the same thing--by years of ten months implies a tacit, but not to be misunderstood, confession of the irregularity and untrustworthiness of the oldest Roman solar year. This Roman calendar may be regarded, at least in its essential features, as that generally current among the Latins.
When we consider how generally the beginning of the year and the names of the months are liable to change, minor variations in the numbering and designations are quite compatible with the hypothesis of a common basis; and with such a calendar-system, which practically was irrespective of the lunar course, the Latins might easily come to have their months of arbitrary length, possibly marked off by annual festivals--as in the case of the Alban months, which varied between 16 and 36 days.
It would appear probable therefore that the Greek τριετηρίς had early been introduced from Lower Italy at least into Latium and perhaps also among the other Italian stocks, and had thereafter been subjected in the calendars of the several cities to further subordinate alterations.
For the measuring of periods of more than one year the regnal years of the kings might have been employed: but it is doubtful whether that method of dating, which was in use in the East, occurred in Greece or Italy during earlier times. On the other hand the intercalary period recurring every four years, and the census and lustration of the community connected with it, appear to have suggested a reckoning by -lustra- similar in plan to the Greek reckoning by Olympiads--a method, however, which early lost its chronological significance in consequence of the irregularity that now prevailed as to the due holding of the census at the right time.
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Reference address : https://www.ellopos.net/elpenor/rome/1-14-measuring-writing.asp?pg=15