 So we know that he relied heavily on "De Excidio et Conquestu Britanniae" by Gildas, a sixth-century cleric, for his early dates and historians have found Gildas unreliable where dates were concerned.Bede's work was widely read among the literate in the Anglo-Saxon kingdoms, and his dates were used by the monks who compiled the various Anglo-Saxon Chronicles from the late ninth century onwards.The most controversial dates in the period—those from the fourth to the late sixth centuries—have very few contemporary sources, and are mainly derived from later attempts to construct Anglo-Saxon history.