The Chartres recension of the Historia Brittonum mentions that Slebine, abbot of Iona from 752 to 767, visited Ripon in Northumbria and found there a dating of the Adventus.
“Et in tempore Guorthigirni regis Britanie Saxones peruenerunt in Britanniam, id est in anno incarnacionis Chisti, sicut Libine abas Iae in Ripum ciuitate inuenit uel reperit. Ab incarnacione Domini anni .d. usque a kł. ian̄. in .xii. luna, ut a[i]unt alii in .ccctis. annis a quo tenuerunt Saxones Britanniam usque ad annum supradictum.”
The dates for his visit and that of the arrival of the English indicated by Grosjean are 753 and 453 respectively. This will be shown to be confirmed by lunar cycles. The first point that needs clarification is which computus was being used in the above quote.
Using the Victorian computus the epact, that is the phase of the moon on the 1st January, was xxii for the year 753, not xii as required by the quote:
However, the Dionysiac computus, which uses the age of the Moon on the 22 March, the epact for the year 753 is indeed xii:
Thus the year of Slébíne’s visit to Ripon was indeed 753. Further confirmation of this is provided by the letter “d” in the quote. In the past this has been wrongly interpreted as the number 500 since it immediately follows “anni”. However, in this context that interpretation would have no meaning. In fact, it represents the word ‘Dominicus’, that is Sunday, which indeed was the day on the 22 March. The original text would have read “usque a .d. kł. ian̄. in .xii. luna”. “The “.ccctis. annis”, therefore, indicates an AS 300 years earlier at 453.
The Annals of Ulster under the year 464:
“The Angles came to England.”
The gap between Chartres 453 and the AU 464 date for the Adventus is as a result of a difference of a hendecad.
 Grosjean, P., 1960, Analecta Bollandiana 78, 381.