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
In like manner this calendar departed from the
original division of the month into four weeks, sometimes of 7,
sometimes of 8 days; it made the eight-day-week run on through the
years without regard to the other relations of the calendar, as our
Sundays do, and placed the weekly market on the day with which it
began (-noundinae-).
Along with this it once for all fixed the
first quarter in the months of 31 days on the seventh, in those
of 29 on the fifth day, and the full moon in the former on the
fifteenth, in the latter on the thirteenth day. As the course of
the months was thus permanently arranged, it was henceforth necessary
to proclaim only the number of days lying between the new moon and
the first quarter; thence the day of the newmoon received the name
of "proclamation-day" (-kalendae-).
The first day of the second
section of the month, uniformly of 8 days, was--in conformity with
the Roman custom of reckoning, which included the -terminus ad
quem- --designated as "nine-day" (-nonae-). The day of the full
moon retained the old name of -idus- (perhaps "dividing-day").