Wikipedia lays out the phantom time hypothesis, the odd belief that certain eras of history did not occur:
The Phantom Time Hypothesis is a conspiracy theory developed by Heribert Illig in 1991. It proposes that periods of history, specifically that of Europe during the Early Middle Ages (AD 614–911), did not exist, and that there has been a systematic effort to cover up that fact. Illig believed that this was achieved through the alteration, misrepresentation and forgery of documentary and physical evidence.
The bases of Illig’s hypothesis include:
The scarcity of archaeological evidence that can be reliably dated to the period AD 614–911, on perceived inadequacies of radiometric and dendrochronological methods of dating this period.
The presence of Romanesque architecture in tenth-century Western Europe. This is taken as evidence that less than half a millennium could have passed since the fall of the Roman Empire, and concludes that the entire Carolingian period, including the person of Charlemagne, is a forgery by medieval chroniclers, more precisely a conspiracy instigated by Otto III and Gerbert d’Aurillac.
The discrepancy between the Julian calendar (introduced by Julius Caesar) and the real (or tropical) calendar. By the time the modern Gregorian calendar was introduced in AD 1582, the imprecise Julian calendar “should” have produced a discrepancy of thirteen days between it and the real calendar. Instead, the astronomers and mathematicians working for Pope Gregory had found that the civil calendar needed to be adjusted by only ten days. From this, Illig concludes that the AD era had counted roughly three centuries which never existed.