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
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Limitation of the Dictatorship
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.
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Reference address : https://www.ellopos.net/elpenor/rome/2-03-equalization-orders-aristocracy.asp?pg=48