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
To all appearance the Celtic nation, when Caesar
encountered it, had already reached the maximum of the culture
allotted to it, and was even now on the decline. The civilization
of the Transalpine Celts in Caesar's time presents, even for us
who are but very imperfectly informed regarding it, several aspects
that are estimable, and yet more that are interesting; in some
respects it is more akin to the modern than to the Greek-Roman
culture, with its sailing vessels, its knighthood, its ecclesiastical
constitution, above all with its attempts, however imperfect,
to build the state not on the city, but on the tribe and in a higher
degree on the nation.
But just because we here meet the Celtic nation
at the culminating point of its development, its lesser degree
of moral endowment or, which is the same thing, its lesser
capacity of culture, comes more distinctly into view.
It was unable to produce from its own resources either a national
art or a national state; it attained at the utmost a national theology
and a peculiar type of nobility.