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
The census lists of the Roman burgesses furnished the commentary on these words. From the end of the Hannibalic war down to 595 the numbers of the burgesses were steadily on the increase, the cause of which is mainly to be sought in the continuous and considerable distributions of domain-land:(22) after 595 again, when the census yielded 328,000 burgesses capable of bearing arms, there appears a regular falling-off, for the list in 600 stood at 324,000, that in 607 at 322,000, that in 623 at 319,000 burgesses fit for service--an alarming result for a time of profound peace at home and abroad.
22. Cf. III. XII. Falling Off of the Population
If matters were to go on at this rate, the burgess-body would resolve itself into planters and slaves; and the Roman state might at length, as was the case with the Parthians, purchase its soldiers in the slave-market.
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Reference address : https://www.ellopos.net/elpenor/rome/4-02-reform-tiberius-gracchus.asp?pg=29