I always think that is such a weird party trick that it is strange that savants develop it ob their own. You have to be a genius to do that without a structured system - but everyone can do that given an hour or two to remember the first day of the week for every decade, the number of days oer quarter%7, etc.

The guys who wrote Peak are probably sub-geniuses then, because they think some savants have figured out a system. In the book they call it mental representation. It's the obsessive ones who develop the skill, some just remain autistic and that's it. They would say it is our bias and desire to believe in magic that predispose us to believe these people are just gifted.

I met someone like this once, he could also tell you the phase of the moon on the day you were born as well as all the celebrities born that day. It was pretty striking.

It's pretty striking you could tell he was right :)