Determining the difference between dates

Question

I'm trying to figure out a way for my program to take a date (like February 2nd, 2003) and show the difference between the two with another date (like April 2nd, 2012), excluding leap years. So far I've only been able to figure it out if the dates are in the same month, just by subtracting the "day". In this program I use 2 sets of "month", "day" and "year" integers. I'm pretty much at a loss from where to go from here. This is a completely optional part of my assignment but I'd like to get an idea on how to get it to work. It seems like a hassle to me, but maybe there's a simple math formula I'm not thinking about?

Sorry, I don't have any pre-existing code for this part because the rest of the assignment just deals with having the user enter dates and then adding and subtracting a single day.

Answer 1

New answer for an old question:

chrono-Compatible Low-Level Date Algorithms

has formulas for converting a {year, month, day} triple to a serial count of days and back. You can use it to calculate the number of days between two dates like this:

std::cout << days_from_civil(2012, 4, 2) - days_from_civil(2003, 2, 2) << '\n';

which outputs:

3347

The paper is a how-to manual, not a library. It uses C++14 to demonstrate the formulas. Each formula comes with a detailed description and derivation, that you only have to read if you care about knowing how the formula works.

The formulas are very efficient, and valid over an extremely large range. For example using 32 bit arithmetic, +/- 5 million years (more than enough).

The serial day count is a count of days since (or prior to for negative values) New Years 1970, making the formulas compatible with Unix Time and all known implementations of std::chrono::system_clock.

The days_from_civil algorithm is not novel, and it should look very similar to other algorithms for doing the same thing. But going the other way, from a count of days back to a {year, month, day} triple is trickier. This is the formula documented by civil_from_days and I have not seen other formulations that are as compact as this one.

The paper includes example uses showing typical computations, std::chrono interoperability, and extensive unit tests demonstrating the correctness over +/- 1 million years (using a proleptic Gregorian calendar).

All of the formulas and software are in the public domain.