The year of hell we've just lived through was actually 1723, according to the Phantom Time Hypothesis. As you were saying "let's hope 2021 is better" you should have been saying "bring on 1724" according to those who believe it.
In the 1990s, several German historians proposed a radical idea: The Middle Ages didn't exist, and we are in fact living through the 18th Century. Let's go through their undeniably fun claims, to begin with, before moving on to their – ahem – "merit".
In 1582, Pope Gregory XIII introduced the Gregorian calendar. Prior to this, most of the Roman world and Europe had used the Julian calendar, proposed by Julius Caesar in 48 BC. In order to try and link the calendar up to the solar year (the time it takes for Earth to revolve around the Sun), the Julian calendar had a leap year every four years, in which an extra day was added.
The problem was that since the actual solar year is 365.24219 days, the Julian calendar quickly (and by quickly, I mean incredibly slowly) gains a day every 128 years. To account for this, when the Gregorian calendar was introduced, Pope Gregory should have shifted the date by 13 days – yet after his calculations, he only shifted it by 10.
Here we have two possibilities: A pope had done the math wrong, or 300 years in between then and now didn't actually exist, and large chunks of history (including Charlemagne) had been fabricated for tenuous purposes. Both would account for the three days, but clearly, the latter is more fun.
First outlined by Heribert Illig in 1991, before being expanded upon in 1995 by Dr Hans-Ulrich Niemitz, the phantom time hypothesis claims that there is far more evidence than merely a pope who didn't pay attention in astronomy class. Niemitz, in a paper on the subject, points to a lack of progress in the west during this time, the lack of records of big events such as the spread of Islam through the Middle East, and – he asserts – the sudden disappearance and then reappearance of Jewish people between 711-1096 AD.
He also points to architecture which he believes to be anachronistic.
"One of the best examples, intensively surveyed, is the Chapel of Aachen (ca. 800 AD), which seems to come approximately 200 years too early," Niemitz writes. "The way of constructing an arch shown in this chapel has no predecessor."
Buildings of similar design didn't come, he claims, until several centuries later in 1049 AD. For him, the best explanation for this – and other questions, such as why the idea of purgatory did not evolve much between 600 and 1100 AD – is that virtually no time had actually passed, but hundreds of years had been inserted into the calendar in the interim.
Who would do this, why, and how?
Niemitz proposed that Otto III, Holy Roman Emperor, wanted to live in the year 1000 AD "because this suited his understanding of Christian milleniarism", but had been born hundreds of years too early. He and the pope (there's always a pope) got together and defined the date as 1000 AD, leaving a backlog of around 300 years to be filled up by chroniclers.
In this version of events, Otto made up Charlemagne "as the model hero he himself wanted to be", before centuries later others embellished Charlemagne's story, while others added in their own made-up history during this period.
It all sounds fun. Who doesn't want to suddenly find themselves in 1724 and enjoy doing what they loved to do back then, such as [checks notes] dying of smallpox? It is, however, hotly contested nonsense.
As well as being able to disprove it through looking at history from around the world at the time (the Tang Dynasty of China etc sync up just fine with the middle ages, without needing any phantom time) and the fact that a conspiracy would have required a wholly unrealistic amount of cooperation and cover-up. Humanity has incredibly good records of astronomical events, and they can disprove the theory on their own.
Roman philosopher and naturalist Pliny the Elder recorded an eclipse in his writings, dating it to 59 AD. Through our own observations, we place an eclipse on that day too – based on our own dates, not by removing 300 years on a whim.
What's more, Chinese records show every appearance of Halley's Comet since 240 B.C, including passes during the "phantom" time, all of which can be verified with our own astronomical calculations today.
Sorry, phantom time fans, the year is 2021 and a pope was bad at math.