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
But want of mathematical precision and theological
scruples, especially in reference to the annual festival of Terminus
which fell within those very days in February, disarranged the
intended reform, so that the Februaries of the intercalary years came
to be of 24 and 23 days, and thus the new Roman solar year in reality
ran to 366 1/4 days. Some remedy for the practical evils resulting
from this was found in the practice by which, setting aside the
reckoning by the months or ten months of the calendar (30) as now no
longer applicable from the inequality in the length of the months,
wherever more accurate specifications were required, they accustomed
themselves to reckon by terms of ten months of a solar year of 365
days or by the so-called ten-month year of 304 days.
Over and above
this, there came early into use in Italy, especially for agricultural
purposes, the farmers' calendar based on the Egyptian solar year of
365 1/4 days by Eudoxus (who flourished 386).