Contradictioning the Theory

Despite the theory's plucky underdog quality it is has a truly phenomenal number of critical flaws, sorry...

Astronomical observations of the sky and Halley's comet from the 'fabricated'  era coincide with our calculations of when Halley's comet would've appeared in history, so either the archetypes of this fabricated era took even comets into account, or the theory is not entirely true...

Halley's comet

Most reputable dating techniques from reputable archaeologists ,such as carbon and uranium dating show that supposed artefacts from 614-911 are genuine and allow for our current timeline.

The date difference between the two calenders is in fact a myth which appears to have spawned from a bizarre equinox in 45BC that caused confusion about the calenders - as simple as that, people,getting confused about a calender.

Julian calendar 

Because the main focus of the theory is Europe then the history of other areas of the world during the same time period are affected by a knock-on effect. The widely-evidenced Tang Dynasty of China, beginnings of Islam and other major world history would have been impossible.

Tang Dynasty map

For more faults of the theory see "The Maths Behind the Theory"