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
But they were not the only dwellers on the hills by the bank of the Tiber. In the earliest division of the burgesses of Rome a trace has been preserved of the fact that that body arose out of the amalgamation of three cantons once probably independent, the Ramnians, Tities, and Luceres, into a single commonwealth--in other words, out of such a --synoikismos-- as that from which Athens arose in Attica.(2) The great antiquity of this threefold division of the community(3) is perhaps best evinced by the fact that the Romans, in matters especially of constitutional law, regularly used the forms -tribuere- ("to divide into three") and -tribus- ("a third") in the general sense of "to divide" and "a part," and the latter expression (-tribus-), like our "quarter," early lost its original signification of number. After the union each of these three communities--once separate, but now forming subdivisions of a single community--still possessed its third of the common domain, and had its proportional representation in the burgess-force and in the council of the elders. In ritual also, the number divisible by three of the members of almost all the oldest colleges--of the Vestal Virgins, the Salii, the Arval Brethren, the Luperci, the Augurs-- probably had reference to that three-fold partition. These three elements into which the primitive body of burgesses in Rome was divided have had theories of the most extravagant absurdity engrafted upon them. The irrational opinion that the Roman nation was a mongrel people finds its support in that division, and its advocates have striven by various means to represent the three great Italian races as elements entering into the composition of the primitive Rome, and to transform a people which has exhibited in language, polity, and religion, a pure and national development such as few have equalled, into a confused aggregate of Etruscan and Sabine, Greek and, forsooth! even Pelasgian fragments.
2. The --synoikismos-- did not necessarily involve an actual settlement together at one spot; but while each resided as formerly on his own land, there was thenceforth only one council-hall and court-house for the whole (Thucyd. ii. 15; Herodot. i. 170).
3. We might even, looking to the Attic --trittus-- and the Umbrian -trifo-, raise the question whether a triple division of the community was not a fundamental principle of the Graeco-ltalians: in that case the triple division of the Roman community would not be referable to the amalgamation of several once independent tribes. But, in order to the establishment of a hypothesis so much at variance with tradition, such a threefold division would require to present itself more generally throughout the Graeco-Italian field than seems to be the case, and to appear uniformly everywhere as the ground-scheme. The Umbrians may possibly have adopted the word -tribus- only when they came under the influence of Roman rule; it cannot with certainty be traced in Oscan.
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Reference address : https://www.ellopos.net/elpenor/rome/1-04-beginnings-rome.asp?pg=2