I am asked to find on which day of the week a specific date falls, from a base date. The first part is for one particular date (14th of July 1789, from 14th of July 2012 which we know is a saturday. I found it is a Tuesday). In the second part, I need to generalize my method to any date.

They tell me I need the number of days between date 1 and 2, but for that i need to know if the year is a leap year or a "normal" year. A leap year is either divisible by 400 or divisible by 4 w/o being divisible by 100.

The book proposes that, in order to find how many years are divisible by 400 between year 1 and year A, we need to take the quotient of the Euclidean Division of (A-1) by 400 so (A-1) = 400q r. q being the number of years divisible by 400.

I didn't understand it at first but later thought of it this way: if q shows the number of times 400 goes into (A-1) , then it also shows how many numbers forming (A-1) it can divide.

Does anyone have a better way of explaining it? Am I correct?