I have my own ideas about this but welcome the thoughts of people here. In particular, I am interested in how ordinary people in the early middle ages actually recorded years. Did they really rely on priests to tell them the time (it is essential to the hypothesis that the whole Church was involved in the conspiracy)? Or did they record dates and years in their own way? How often did official documents record the exact date?
My only knowledge of reference to dates is Bede, who includes a whole chronology of the world in his book on the English Church and people. (However, Bede was living right in the middle of the 'phantom period' so perhaps his works were a later forgery).