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
The first Roman officer who entered Brittany, Publius Crassus had already (697) crossed thence to the "tin-islands" at the south-west point of England (Stilly islands); in the summer of 699 Caesar himself with only two legions crossed the Channel at its narrowest part.(42)
42. The nature of the case as well as Caesar's express statement proves that the passages of Caesar to Britain were made from ports of the coast between Calais and Boulogne to the coast of Kent. A more exact determination of the localities has often been attempted, but without success. All that is recorded is, that on the first voyage the infantry embarked at one port, the cavalry at another distant from the former eight miles in an easterly direction (iv. 22, 23, 28), and that the second voyage was made from that one of those two ports which Caesar had found most convenient, the (otherwise not further mentioned) Portus Itius, distant from the British coast 30 (so according to the MSS. of Caesar v. 2) or 40 miles (=320 stadia, according to Strabo iv. 5, 2, who doubtless drew his account from Caesar).
From Caesar's words (iv. 21) that he had chosen "the shortest crossing," we may doubtless reasonably infer that he crossed not the Channel but the Straits of Calais, but by no means that he crossed the latter by the mathematically shortest line. It requires the implicit faith of local topographers to proceed to the determination of the locality with such data in hand--data of which the best in itself becomes almost useless from the variation of the authorities as to the number; but among the many possibilities most may perhaps be said in favour of the view that the Itian port (which Strabo l. c. is probably right in identifying with that from which the infantry crossed in the first voyage) is to be sought near Ambleteuse to the west of Cape Gris Nez, and the cavalry-harbour near Ecale (Wissant) to the east of the same promontory, and that the landing took place to the east of Dover near Walmer Castle.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=92