computing the day difference between two dates

datediff.c

/* this code is from http://c-faq.com/lib/calendar.br.html */

#define DayOfWeek(d,m,y) (int)(DaysElapsed(d,m,y) % 7)

long DaysElapsed(int d,int m,int y) {
    static int cd[]={0,31,59,90,120,151,181,212,243,273,304,334};
    long n=365L*(y-1);

    if (m<3) y--;
    return n+y/4-y/100+y/400+cd[m-1]+d;
}
difference=DaysElapsed(d2,m2,y2)-DaysElapsed(d1,m1,y1);

/**
* Here are some long explanations:
* http://alcor.concordia.ca/~gpkatch/gdate-algorithm.html
* http://alcor.concordia.ca/~gpkatch/gdate-method.html
*/