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
Legislation of the Twelve Tables
The plan, if it should stand, was a wise one; all depended on whether men's minds exasperated on either side with passion would accept that peaceful adjustment. The decemvirs of the year 303 submitted their law to the people, and it was confirmed by them, engraven on ten tables of copper, and affixed in the Forum to the rostra in front of the senate-house. But as a supplement appeared necessary, decemvirs were again nominated in the year 304, who added two more tables.
Thus originated the first and only Roman code, the law of the Twelve Tables. It proceeded from a compromise between parties, and for that very reason could not well have contained any changes in the existing law of a comprehensive nature, going beyond the regulation of secondary matters and of the mere adaptation of means and ends. Even in the system of credit no further alleviation was introduced than the establishment of a--probably low--maximum of interest (10 per cent) and the threatening of heavy penalties against the usurer-penalties, characteristically enough, far heavier than those of the thief; the harsh procedure in actions of debt remained at least in its leading features unaltered.
Still less, as may easily be conceived, were changes contemplated in the rights of the orders. On the contrary the legal distinction between burgesses liable to be taxed and those who were without estate, and the invalidity of marriage between patricians and plebeians, were confirmed anew in the law of the city. In like manner, with a view to restrict the caprice of the magistrate and to protect the burgess, it was expressly enacted that the later law should uniformly have precedence over the earlier, and that no decree of the people should be issued against a single burgess.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=32