Efficient algorithm for converting number of days to years (including leap years)

Question

The problem

I am writing a class for holding dates in c++, and I found the following problem:

I have a number of days N since a reference date (in

Answer 1

To use the punchline of an old joke, "I wouldn't start from here".

You'll want to read up about various changes to calendaring before "modern" times, for example, what happened in 1752