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 Tribunate of the People as an Instrument of Government
While the Roman magistrate was thus more and more completely and definitely transformed from the absolute lord into the limited commissioner and administrator of the community, the old counter-magistracy, the tribunate of the people, was undergoing at the same time a similar transformation internal rather than external. It served a double purpose in the commonwealth.
It had been from the beginning intended to protect the humble and the weak by a somewhat revolutionary assistance (-auxilium-) against the overbearing violence of the magistrates; it had subsequently been employed to get rid of the legal disabilities of the commons and the privileges of the gentile nobility. The latter end was attained.
The original object was not only in itself a democratic ideal rather than a political possibility, but it was also quite as obnoxious to the plebeian aristocracy into whose hands the tribunate necessarily fell, and quite as incompatible with the new organization which originated in the equalization of the orders and had if possible a still more decided aristocratic hue than that which preceded it, as it was obnoxious to the gentile nobility and incompatible with the patrician consular constitution.
But instead of abolishing the tribunate, they preferred to convert it from a weapon of opposition into an instrument of government, and now introduced the tribunes of the people, who were originally excluded from all share in administration and were neither magistrates nor members of the senate, into the class of governing authorities.
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Reference address : https://www.ellopos.net/elpenor/rome/2-03-equalization-orders-aristocracy.asp?pg=50