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 speed up the calculation of the year, you could build a lookup table

int[] YearStartDays =
{
    0,                     // 1 AD
    365,                   // 2 AD
    365 + 365,             // 3 AD
    365 + 365 + 365,       // 4 AD
    365 + 365 + 365 + 366, // 5 AD (4 was a leap year)
    /* ... */
};

You can then do a binary search in this array, which is O(log N) instead of the O(N) of your current year finding algorithm.