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
It was not till Manius Valerius was nominated dictator that the farmers submitted, partly from their awe of the higher magisterial authority, partly from their confidence in his friendly feeling to the popular cause--for the Valerii were one of those old patrician clans by whom government was esteemed a privilege and an honour, not a source of gain. The victory was again with the Roman standards; but when the victors came home and the dictator submitted his proposals of reform to the senate, they were thwarted by its obstinate opposition.
The army still stood in its array, as usual, before the gates of the city. When the news arrived, the long threatening storm burst forth; the -esprit de corps- and the compact military organization carried even the timid and the indifferent along with the movement. The army abandoned its general and its encampment, and under the leadership of the commanders of the legions--the military tribunes, who were at least in great part plebeians--marched in martial order into the district of Crustumeria between the Tiber and the Anio, where it occupied a hill and threatened to establish in this most fertile part of the Roman territory a new plebeian city.
This secession showed in a palpable manner even to the most obstinate of the oppressors that such a civil war must end with economic ruin to themselves; and the senate gave way. The dictator negotiated an agreement; the citizens returned within the city walls; unity was outwardly restored. The people gave Manius Valerius thenceforth the name of "the great" (-maximus-)--and called the mount beyond the Anio "the sacred mount." There was something mighty and elevating in such a revolution, undertaken by the multitude itself without definite guidance under generals whom accident supplied, and accomplished without bloodshed; and with pleasure and pride the citizens recalled its memory. Its consequences were felt for many centuries: it was the origin of the tribunate of the plebs.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=11