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 by the side of the king and of the burgess-assembly there appears in the earliest constitution of the community a third original power, not destined for acting like the former or for resolving like the latter, and yet co-ordinate with both and within its own rightful sphere placed over both. This was the council of elders or -senatus-.
Beyond doubt it had its origin in the clan-constitution: the old tradition that in the original Rome the senate was composed of all the heads of households is correct in state-law to this extent, that each of the clans of the later Rome which had not merely migrated thither at a more recent date referred its origin to one of those household-fathers of the primitive city as its ancestor and patriarch.
If, as is probable, there was once in Rome or at any rate in Latium a time when, like the state itself, each of its ultimate constituents, that is to say each clan, had virtually a monarchical organization and was under the rule of an elder--whether raised to that position by the choice of the clansmen or of his predecessor, or in virtue of hereditary succession--the senate of that time was nothing but the collective body of these clan-elders, and accordingly an institution independent of the king and of the burgess-assembly; in contradistinction to the latter, which was directly composed of the whole body of the burgesses, it was in some measure a representative assembly of persons acting for the people.
Certainly that stage of independence when each clan was virtually a state was surmounted in the Latin stock at an immemorially early period, and the first and perhaps most difficult step towards developing the community out of the clan-organization--the setting aside of the clan-elders--had possibly been taken in Latium long before the foundation of Rome; the Roman clan, as we know it, is without any visible head, and no one of the living clansmen is especially called to represent the common patriarch from whom all the clansmen descend or profess to descend so that even inheritance and guardianship, when they fall by death to the clan, devolve on the clan-members as a whole.
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Reference address : https://www.ellopos.net/elpenor/rome/1-05-original-constitution-rome.asp?pg=27