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
Like a crust
of ice gathering imperceptibly over the surface of a stream and
imperceptibly confining it more and more, this new Roman aristocracy
silently arose; and not less imperceptibly, like the current
concealing itself beneath and slowly extending, there arose in
opposition to it the new party of progress.
It is very difficult
to sum up in a general historical view the several, individually
insignificant, traces of these two antagonistic movements, which do
not for the present yield their historical product in any distinct
actual catastrophe.
But the freedom hitherto enjoyed in the
commonwealth was undermined, and the foundation for future revolutions
was laid, during this epoch; and the delineation of these as well as
of the development of Rome in general would remain imperfect, if we
should fail to give some idea of the strength of that encrusting ice,
of the growth of the current beneath, and of the fearful moaning and
cracking that foretold the mighty breaking up which was at hand.