The Date of the Battle of Badon Controversy
The The date of the battle of Badon has long been the subject of controversy among students of the period. The uncertainty is due to the ambiguity of Gildas’s words in his book De Excidio et Conquestu Britanniae. He refers in the context of Badon to “the 44th year with one month elapsed”, i.e. a period of 43 years and one month. But it is not linguistically clear whether the period was before the Battle of Badon – between a previous event mentioned in his book and the battle – or after it, between the battle and the date of writing.
How to Interpret Gildas
A major difficulty with interpreting this as the period between the battle and the time of writing is: how could Gildas while he was still writing have known to the month at what date his work would be finished? The alternative that the 43 years and one month ended with the battle (which of course did have an exact date) leads to the question: which of three events Gildas discussed in the preceding chapters of De Excidio he intended as the starting point. It could reasonably be any of: the letter to Aetius (Chapter 20); the council of Britain deciding to invite some Saxons to fight for Britain (Chapter 23); or the coming to power of Ambrosius (Chapter 25).
Gildas could reasonably have known the month and year of any of these. But the coming to power of Ambrosius should be ruled out, because Gildas indicates that it happened after and in consequence of the Great Raid of 473. This would require the date of Badon to be 516 or up to a few years later. A date as late as this is inconsistent with Gildas’s saying that the people who were in positions of responsibility during the warring that culminated at Badon, including kings, officials and priests, had all died by the time of writing. De Excidio was published during the lifetime of King Maelgwn (Mailcun in the older spelling used in the Annales Cambriae [AC]), so not later than 548CE. A period of thirty years or so is not sufficient for this to be true. Fifty years at least would be necessary.
Also, Gildas records that he was born in the year of the battle of Badon. Hagiographical writings record that he gave or sent a bell to St. Bride. (It is presumed that he was a bell-maker.) If true, this has to have been before her death in 524, and therefore Gildas’s birth can hardly have been later than, at a squeeze, 510. There is also the tradition that he founded Rhuys Abbey in the 520s. This date is not as assured as that of St. Bride’s death – but even if the foundation date were in the 530s it would speak in favour of a date of Gildas’s birth earlier than 516.
43 Years after Aetius or Hengist
It is much more probable therefore that Gildas intended us to understand his period of 43 years and 1 month to begin with one of the other two seminal events he reports, the letter to Aetius or the invitation to Hengist. The letter to Aetius is the most probable, as it is the only one of the three events in writing, and therefore with an exact date on its face. It is figured to have been sent in 446 because that was the year of his third consulship, to which the letter refers.
In any case though, if the council of Britain was meant it makes little difference to the calculation for the date of Badon. The council cannot have been long after the letter, for response to the emergency prompting the request to Aetius for military aid was urgent. It makes logical sense to postulate that the council convened late in 446 or early in 447, the year when according to Mageoghagan’s Irish annals Hengist actually arrived with his English warriors in response to that council’s request. On either basis, the resulting calculation is that the Battle of Badon was fought in 489 or 490.
Badon in 490 CE Makes Sense
This date for Badon and therefore for Gildas’s birth fits the information about Gildas’s life excellently. It is also fully consistent with the archaeological record, which shows a break in the English penetration of Britain approximately comprising the first half of the sixth century; and with the Anglo-Saxon Chronicle which gives no battles other than in Wessex between 490 and 550. Its supposed battles in Wessex have been interpreted as an ‘origin myth’ for the Gewissae dynasty, rather than factual truth.
The Annales Cambriae Date for Badon can be Amended to 490
It can be reconciled with the AC on a straightforward hypothesis: that the original Christian source said the Battle of Badon was “490 years after the Incarnation of Christ”, and that an early copyist mis-transcribed that as “490 years after the Passion of Christ’. Scribes before the introduction of the AD calendar sometimes expressed dates anno passio, and 27 needs to be subtracted from the number thus given to give the corresponding CE year. The AC date for the Battle of Badon on a +445 calibration is 517CE, and 517 minus 27 is 490.
490CE as the date of the Battle of Badon fits well with all the evidence. It does not require the AC compilers to have made a complex derivation of the date of Badon, such as by interpreting De Excidio and the Anglo-Saxon Chronicle while not using other dates also found in the same sources. It is compatible with the Anglo-Saxon Chronicle, Bede, the archaeological evidence, the floruit of Ambrosius, the Irish annals, De Excidio, the events of Gildas’s life, and the other entries in the AC.
The 27-year Passion/Incarnation mistake does nothing to diminish the likelihood of authenticity of the AC entry. On the contrary, it enhances it, for the error is much more probable with a source that was originally written before the invention of the AD calendar in 529.
 On the calibration of the AC to the CE calendar by adding 445 to the AC’s internal year ticker count, which I advocate for its pre-565 entries (see my previous post here of 21 September 2013).
 The Historia Brittonum demonstrates that such mistakes happened: Chapters 16 and 66 show a similar mistake in reverse, 405 and 400 years after the Incarnation, respectively, being written when “years after the Passion” should have been written.