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
Historical Composition - Polybius
In historical composition this epoch is especially marked by the emergence of an author who did not belong to Italy either by birth or in respect of his intellectual and literary standpoint, but who first or rather alone brought literary appreciation and description to bear on Rome's place in the world, and to whom all subsequent generations, and we too, owe the best part of our knowledge of the Roman development.
Polybius (c. 546-c. 627) of Megalopolis in the Peloponnesus, son of the Achaean statesman Lycortas, took part apparently as early as 565 in the expedition of the Romans against the Celts of Asia Minor, and was afterwards on various occasions, especially during the third Macedonian war, employed by his countrymen in military and diplomatic affairs. After the crisis occasioned by that war in Greece he was carried off along with the other Achaean hostages to Italy,(26) where he lived in exile for seventeen years (587-604) and was introduced by the sons of Paullus to the genteel circles of the capital.
26. Cf. III. X. Measures of Security in Greece
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Reference address : https://www.ellopos.net/elpenor/rome/4-13-literature-art.asp?pg=37