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
The old unity and plenary legal power of the -imperium- were retained
longer in the case of the dictatorship than in that of the consulship.
Although of course as an extraordinary magistracy it had in reality
from the first its special functions, it had in law far less of a
special character than the consulate. But it also was gradually
affected by the new idea of definite powers and functions introduced
into the legal life of Rome.
In 391 we first meet with a dictator
expressly nominated from theological scruples for the mere
accomplishment of a religious ceremony; and though that dictator
himself, doubtless in formal accordance with the constitution,
treated the restriction of his powers as null and took the command
of the army in spite of it, such an opposition on the part of the
magistrate was not repeated on occasion of the subsequent similarly
restricted nominations, which occurred in 403 and thenceforward very
frequently. On the contrary, the dictators thenceforth accounted
themselves bound by their powers as specially defined.