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
They had, moreover, long ago brought in the Italian allies to share to a very great extent in war-service; but in their case too, just as among the Roman burgesses, military duty was chiefly imposed on the propertied classes. Nevertheless the Roman military system down to the time of Marius rested in the main on that primitive organization of the burgess-militia. But it was no longer suited for the altered circumstances.
The better classes of society kept aloof more and more from service in the army, and the Roman and Italic middle class in general was disappearing; while on the other hand the considerable military resources of the extra-Italian allies and subjects had become available, and the Italian proletariate also, properly applied, afforded at least a very useful material for military objects. The burgess- cavalry,(4) which was meant to be formed from the class of the wealthy, had practically ceased from service in the field even before the time of Marius.
4. Cf. III. XI. The Nobility in Possession of the Equestrian Centuries
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Reference address : https://www.ellopos.net/elpenor/rome/4-06-marius-revolution-drusus-reform.asp?pg=8