The Historia Brittonum contains a number of statements which allow us to date the Adventus Saxonum:
“Vortigern, however, held power in Britain in the consulate of Theodosius and Valentinian and in the fourth year of his reign the Saxons came to Britain, in the consulship of Felix and Taurus, and in the four hundredth year from the incarnation of our lord Jesus Christ.”
The first consulate of Theodosius II and Valentinian III was in the year 425. That this was the start of Vortigern’s reign is plausible since he was of gen. -3. This would suggest he was probably in his 30s when crowned king. The passage suggests the Adventus occurred in the year 428 which was, indeed, the year of the consulship of Flavius Felix and Flavius Taurus. The last element of the text may be reconciled with this date by suggesting incarnation was an error for Christ’s Passion.
In the Chronicon Britannicum we see the Adventus incorrectly placed one Metonic cycle later in the year 447, see The Adventus Saxonum in the Chronicon Britannicum. The events of 425 and 428 listed in the HB may have been swapped in the mind of the author when he wrote:
“Also, from Stilicho to Valentinian son of Placidia, and the reign of Vortigern 28 years.”
as Flavius Stilicho was consul for the first time in 400.
HB 31 states:
“It came to pass that after this war between the Britons and the Romans, in which the generals were killed, and after the killing of the tyrant Maximus and the end of the Roman Empire in Britain, the Britons went in fear for 40 years.”
Magnus Maximus was executed in 388. 40 years on from that date we arrive at 428, that is at the Adventus, which is described towards the end of the following passage of text:
“Vortigern welcomed them, and handed over to them the island that in their language is called Thanet, in British Ruoihm.”