The Phantom Time Hypothesis is a theory that says that European history between AD 614–911 is a forgery by the Roman Catholic Church. The theory was suggested by Heribert Illig in 1991 and expanded by other theorists.
The original reasoning behind the theory was that when the Gregorian calendar was introduced in 1582, the difference between it and the old Julian calendar should have been 13 days, but the calendar was only “rewound” 10 days forward. These three days roughly correspond to 300 years, which are “missing”. This was later countered with the argumentation that the Gregorian calendar was synchronized not with AD 1 but with AD 325, when the First Council of Nicaea took place, thus accounting for the “missing” three centuries.
Furthermore, Illig claimed that the archaeological finds that can be reliably dated to the period of 7th-10th centuries are very scarce. He also argued that Romanesque architecture (approx. 10th century AD) couldn’t have emerged as late as five centuries after the Fall of Roman Empire (which happened in 5th century AD). He then concluded that Otto III, Holy Roman Emperor conspired with Pope Sylvester II to retroactively invent the entire Carolingian dynasty including Charlemagne. The purpose of this was to support Otto’s royal claims (his Ottonian dynasty succeeded the Carolingians) and simultaneously “lengthen” the Christian tradition, thus ascertaining its superiority over the emerging Islam.
No attempts to synchronize this theory with alternative calenders (e.g. Chinese calender or Islamic calendar) are documented.