n日後

addday.c

#include <stdio.h>
#include <time.h>

struct mydate { int year,month,day; };
int addDay(struct mydate *d, int n)
{
    struct tm birthday = {0,0,0,d->day,d->month,d->year-1900,0,0,0,0,NULL};
    time_t birthday_sec = mktime(&birthday);
    birthday_sec += (n*24*60*60);
    struct tm *pHA = localtime( &birthday_sec );
    if (pHA == NULL) return -1;
    d->day = pHA->tm_mday;
    d->month = pHA->tm_mon;
    d->year = pHA->tm_year+1900;
    return 0;
}

#ifdef _TEST
int main() {
    struct mydate birthday = {2011,1,25};
    if (addDay(&birthday, 100) >= 0) {
	printf("%d/%d/%d\n", birthday.year,birthday.month, birthday.day);
    }
    return 0;
}
#endif