finding number of days between two dates in the same year (c++)

This is similar to an interview question I ask prospective hires to code for me. (And for purposes of interview evaluation, they are not allowed to use the built in date/time functions).

And the OP's problem is simplified to measuring the delta in days of dates within the same year - so that makes things easier.

To start, we need a simple function to tell us if the year we are dealing with is a leap year, because at some point in the code, we'll have to deal with leap years. And, leap year is a bit more than just "every four year". But you already know that.

bool isLeapYear(int year)
{
    bool isDivisibleByFour = !(year % 4);
    bool isDivisibleBy100 = !(year % 100);
    bool isDivisibleBy400 = !(year % 400);
    return (isDivisibleBy400) || (isDivisibleByFour && !isDivisibleBy100);
}

And we'll need another helper function to return the number of days in a month and it needs to account for for leap years in February.

int getDaysInMonth(int month, int year)
{
    int days_in_month[12] = { 31,28,31,30,31,30,31,31,30,31,30,31 };

    int result = days_in_month[month-1];
    if ((month == 2) && isLeapYear(year))
    {
        result++;
    }
    return result;
}

Assume in the above code that "January" would be represented by "month == 1" and December is "month == 12". Hence, the [month-1] thing in the array lookup. We would add some parameter validation to the above code, but it should work for purposes of discussion.

Now we have a way to count days in a month, we need to a function that will tell us "how many days since the beginning of the year" for a given month/day/year.

int getDayOfYear(int month, int day, int year)
{
    int count = 0;
    int m = 1;

    while (m != month)
    {
        count += getDaysInMonth(m, year);
        m++;
    }
    count += day - 1;
    return count;
}

The function above will return "0" for (1,1,2015) and "364" for (12,31,2015). Again, parameter validation would be needed for production code.

Now to compute the difference in days between any two days of the same year:

int getDiffOfDaysInSameYear(int month1, int day1, int month2, int day2, int year)
{
    int day_of_year1 = getDayOfYear(month1, day1, year);
    int day_of_year2 = getDayOfYear(month2, day2, year);
    return day_of_year2 - day_of_year1;
}

Let's test it out:

int main()
{
    int x = getDiffOfDaysInSameYear(4,4, 10,24, 2015); // number of days to get to  10/24/2015 from 4/4/2015
    printf("The delta in days between April 4 and October 2015 is: %d days\n", x);
    return 0;
}

Prints out: The delta in days between April 4 and October 2015 is: 203 days

If you want to simplify it to just counting days between months, then just pass in "1" for the days.

int main()
{
    int x = getDiffOfDaysInSameYear(5, 1, 11, 1, 2015);
    printf("The delta in days between May and November is %d\n", x);
    return 0;
}

Prints out: The delta in days between May and November is 184

Hope this helps.