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
With still greater folly these sought to impose upon the Celts by haughty language, and, when this failed, they conceived that they might with impunity violate the law of nations in dealing with barbarians; in the ranks of the Clusines they took part in a skirmish, and in the course of it one of them stabbed and dismounted a Gallic officer. The barbarians acted in this case with moderation and prudence.
They sent in the first instance to the Roman community to demand the surrender of those who had outraged the law of nations, and the senate was ready to comply with the reasonable request. But with the multitude compassion for their countrymen outweighed justice towards the foreigners; satisfaction was refused by the burgesses; and according to some accounts they even nominated the brave champions of their fatherland as consular tribunes for the year 364,(9) which was to be so fatal in the Roman annals.
9. This is according to the current computation 390 B. C.; but, in fact, the capture of Rome occurred in Ol. 98, 1 = 388 B. C., and has been thrown out of its proper place merely by the confusion of the Roman calendar.
Do you see any typos or other mistakes? Please let us know and correct them
Reference address : https://www.ellopos.net/elpenor/rome/2-04-fall-etruscan-celts.asp?pg=21