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
Among the sciences, that of jurisprudence acquired an invaluable basis through the committing to writing of the laws of the city in the years 303, 304. This code, known under the name of the Twelve Tables, is perhaps the oldest Roman document that deserves the name of a book. The nucleus of the so-called -leges regiae- was probably not much more recent. These were certain precepts chiefly of a ritual nature, which rested upon traditional usage, and were probably promulgated to the general public under the form of royal enactments by the college of pontifices, which was entitled not to legislate but to point out the law.
Moreover it may be presumed that from the commencement of this period the more important decrees of the senate at any rate--if not those of the people--were regularly recorded in writing; for already in the earliest conflicts between the orders disputes took place as to their preservation.(24)
24. Cf. II. II. Political Value of the Tribunates, II. II. The Valerio-Horatian Laws
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Reference address : https://www.ellopos.net/elpenor/rome/2-09-art-science.asp?pg=25