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
Had the Romans placed a corps, as they might have done, of 30,000 men thoroughly fresh and ready for action somewhere near Turin, and immediately forced on a battle, the prospects of Hannibal's great plan would have been very dubious; fortunately for him, once more, they were not where they should have been, and they did not disturb the troops of the enemy in the repose which was so greatly needed.(4)
4. The much-discussed questions of topography, connected with this celebrated expedition, may be regarded as cleared up and substantially solved by the masterly investigations of Messrs. Wickham and Cramer. Respecting the chronological questions, which likewise present difficulties, a few remarks may be exceptionally allowed to have a place here.
When Hannibal reached the summit of the St. Bernard, "the peaks were already beginning to be thickly covered with snow" (Pol. iii. 54), snow lay on the route (Pol. iii. 55), perhaps for the most part snow not freshly fallen, but proceeding from the fall of avalanches. At the St. Bernard winter begins about Michaelmas, and the falling of snow in September; when the Englishmen already mentioned crossed the mountain at the end of August, they found almost no snow on their road, but the slopes on both sides were covered with it.
Hannibal thus appears to have arrived at the pass in the beginning of September; which is quite compatible with the statement that he arrived there "when the winter was already approaching" --for συνάπτειν τὴν τῆς πλειάδος δύσιν (Pol. iii. 54) does not mean anything more than this, least of all, the day of the heliacal setting of the Pleiades (about 26th October); comp. Ideler, Chronol. i. 241.
If Hannibal reached Italy nine days later, and therefore about the middle of September, there is room for the events that occurred from that time up to the battle of the Trebia towards the end of December (περὶ χειμερινὰς τροπάς, Pol. iii. 72), and in particular for the transporting of the army destined for Africa from Lilybaeum to Placentia. This hypothesis further suits the statement that the day of departure was announced at an assembly of the army ὑπὸ τὴν ἐαρινὴν ὥραν (Pol. iii. 34), and therefore towards the end of March, and that the march lasted five (or, according to App. vii. 4, six) months.
If Hannibal was thus at the St. Bernard in the beginning of September, he must have reached the Rhone at the beginning of August --for he spent thirty days in making his way from the Rhone thither --and in that case it is evident that Scipio, who embarked at the beginning of summer (Pol. iii. 41) and so at latest by the commencement of June, must have spent much time on the voyage or remained for a considerable period in singular inaction at Massilia.
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Reference address : https://www.ellopos.net/elpenor/rome/3-04-hamilcar-hannibal.asp?pg=53