Proposal
The bases of Illig's hypothesis include:[7][8] The scarcity of archaeological evidence that can be reliably dated to the period AD 614–911, the perceived inadequacies of radiometric and dendrochronological methods of dating this period, and the over-reliance of medieval historians on written sources. The presence of Romanesque architecture in tenth-century Western Europe, suggesting the Roman era was not as long ago as conventionally thought. The relation between the Julian calendar, Gregorian calendar and the underlying astronomical solar or tropical year. The Julian calendar, introduced by Julius Caesar, was long known to introduce a discrepancy from the tropical year of around one day for each century that the calendar was in use. By the time the Gregorian calendar was introduced in AD 1582, Illig alleges that the old Julian calendar should have produced a discrepancy of thirteen days between it and the real (or tropical) calendar. Instead, the astronomers and mathematicians working for Pope Gregory XIII had found that the civil calendar needed to be adjusted by only ten days. (The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582). From this, Illig concludes that the AD era had counted roughly three centuries which never existed.