Heh I have a 40 year calendar as part of a tourist souvenir that I thought was "good for the money"! It's also amazing me that I have kept up with it since 1999 after having moved four times since then!
Calendars are also a favorite trick of autistic "small-savants". Rain Man did a tiny bit of damage to the public impression because he was One in a Million. (Based on Kim Peek). Most "Savants" are far less talented than that. But of those, a bunch of them can do Calendar calculations. I knew one from child days at a summer camp. There was no way to check him, but he was reeling off days of the week confidently, so even if he maybe "missed a leap year" or something, he clearly had the concept-pattern down.
In a sense it's "easy enough to do" - there's only some twelve rules, then you just flip multiples of 7/14/28/100 until you get there.