History of Syncretism & Belief in Pagan Origins
One claim made by the proponents of the Calculations Theory is correct. There is a lot of misinformation out there, and much of it has become popular and easily accessible in the wake of the advent of the internet.
First, to address some of the more famous false claims about Christmas Pagan origins and some of the valid claims about the absence of evidence from those pushing the Calculations theory.
There truly is no evidence of Christmas being tied to Mirtha. While Mithraism was a popular competing cult in early Christianity, the belief that Mithra's birthday was December 25 has no historical documentation that would predate the advent of Christmas itself. Most such evidence dates to after Christmas was established.
There is some evidence to suggest that Mithra may have been viewed as being synonymous with Sol (Invictus), and if valid, then arguing for Sol as the origin point is the proper course of action rather than trying to form an argument from Mithras directly.