I used to be able to do the date/day thing but I can't quite remember how I did it. It was more of a pattern recognition thing than a calculation thing. I am not a math person. At all. Basically the dates move along the days in a pattern but then leap year messes up the pattern. But since leap year has a pattern too it just becomes part of the pattern (just the annoying part) and the difference in the number of years is how many cycles to go through the pattern. I am pretty sure that's how I did it.
I was once considered gifted. But certainly not a savant. Now I have Mom Brain which means I am juggling too much on too little sleep. The preceding paragraph is a great example of my clarity of thought wink