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
For some time the day continued to be among the Italians the smallest, and the month the largest, division of time. It was not until afterwards that they began to distribute day and night respectively into four portions, and it was much later still when they began to employ the division into hours; which explains why even stocks otherwise closely related differed in their mode of fixing the commencement of day, the Romans placing it at midnight, the Sabellians and the Etruscans at noon.
No calendar of the year had, at least when the Greeks separated from the Italians, as yet been organized, for the names for the year and its divisions in the two languages have been formed quite independently of each other. Nevertheless the Italians appear to have already in the pre-Greek period advanced, if not to the arrangement of a fixed calendar, at any rate to the institution of two larger units of time.
The simplifying of the reckoning according to lunar months by the application of the decimal system, which was usual among the Romans, and the designation of a term of ten months as a "ring" (-annus-) or complete year, bear in them all the traces of a high antiquity. Later, but still at a period very early and undoubtedly previous to the operation of Greek influences, the duodecimal system (as we have already stated) was developed in Italy, and, as it derived its very origin from the observation of the fact that the solar period was equal to twelve lunar periods, it was certainly applied in the first instance to the reckoning of time.
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Reference address : https://www.ellopos.net/elpenor/rome/1-14-measuring-writing.asp?pg=9