Note 154 — Chapter III · BRITAIN AFTER THE ROMANS.
Reading edition
Guorthigirnus autem tenuit imperium in Brittannia Theodosio et Valentiniano consulibus et in quarto anno regni sui Saxones ad Brittanniam venerunt, Felice et Tauro consulibus, quadringentesimo anno ab incarnatione Domini nostri Jesu Christi.
Nennius appears to have reckoned 27 years between the incarnation and the passion of Christ. We should probably read ‘a passione’ for ‘ab incarnatione,’ which makes the year equal to 427 or 428.