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
People in earnest could not but discern that the interference of the tribunes in administration and their action as prosecutors had an absolutely pernicious effect; and the only real gain which the tribunate brought to the plebeians was the protection which it afforded against a partial administration of justice, by operating as a sort of court of cassation to check the caprice of the magistrate. Beyond doubt, when the plebeians desired a written code, the patricians replied that in that event the legal protection of tribunes would be superfluous; and upon this there appears to have been concession by both sides.
Perhaps there was never anything definitely expressed as to what was to be done after the drawing up of the code; but that the plebs definitely renounced the tribunate is not to be doubted, since it was brought by the decemvirate into such a position that it could not get back the tribunate otherwise than by illegal means. The promise given to the plebs that its sworn liberties should not be touched, may be referred to the rights of the plebeians independent of the tribunate, such as the -provocatio- and the possession of the Aventine.
The intention seems to have been that the decemvirs should, on their retiring, propose to the people to re-elect the consuls who should now judge no longer according to their arbitrary pleasure but according to written law.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=31