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
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.