Calculating days between two dates in C

My professor never really went over style, to my knowledge... just more of, "This is what the finished product looks like" and sent us on our way.

I know my coding/style are messy - that's my thought process, or understanding of it.

I think the idea is not to change things unnecessarily. If you have a function that does X, it shouldn't do Y if Y doesn't serve any practical purpose towards X, especially changing global values.