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
Writing of History
Down to the period of the Hannibalic war there was no historical composition in Rome; for the entries in the book of Annals were of the nature of records and not of literature, and never made any attempt to develop the connection of events. It is a significant illustration of the peculiarity of Roman character, that notwithstanding the extension of the power of the Roman community far beyond the bounds of Italy, and notwithstanding the constant contact of the noble society of Rome with the Greeks who were so fruitful in literary activity, it was not till the middle of the sixth century that there was felt the need and desire of imparting a knowledge of the deeds and fortunes of the Roman people, by means of authorship, to the contemporary world and to posterity.
When at length this desire was felt, there were neither literary forms ready at hand for the use of Roman history, nor was there a public prepared to read it, and great talent and considerable time were required to create both. In the first instance, accordingly, these difficulties were in some measure evaded by writing the national history either in the mother-tongue but in that case in verse, or in prose but in that case in Greek. We have already spoken of the metrical chronicles of Naevius (written about 550?) and of Ennius (written about 581); both belong to the earliest historical literature of the Romans, and the work of Naevius may be regarded as the oldest of all Roman historical works.
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Reference address : https://www.ellopos.net/elpenor/rome/3-14-literature-art.asp?pg=85