While it is accepted that in 1752 we lost 11 days to align the calendar in use in England to the Gregorian one adopted on the continent, there is a conspiracy theory that the period between 614 ad and 911 ad didn’t exist and the year is actually 1719 ad.
The theory is that Holy Roman Emperor Otto III, Pope Sylvester II and Byzantine Emperor Constantine VII fabricated the dates so that it placed them as heads of their respective seats of power at the time of the Millennium, 1000 ad, therefore ensuring their places at this special time in history.
Problem was it was only the seventh century so they colluded to get chroniclers across Europe to invent and document an extra 300 years.
Theorists argue that the apparent stagnation in ceramics, language and thought as well as the lack of substantial documentary evidence along with the lack of development of architecture and the presence of Roman architecture in tenth-century Western Europe, suggesting that the Roman era was not as long as conventionally thought and shows that this period simply didn’t exist.
The period is characterised by a relative scarcity of historical and other written records at least for some areas of Europe, rendering it obscure to historians hence the name 'the Dark Ages'.