날짜 계산하는 C 코드 짜고 있는데 꽉 막혀서 질문드립니다..

#include <stdio.h>
 
int main (int argc, char **argv)
{
        int i, j, n, k, l, m;
        int gyear, gmon, gday;
        int jyear, jday;
        int lastday[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
        int sumday;
        int day;
        int pday, mday;
 
        sumday = 0;
        jyear = 0;
        jday = 0;
        pday = 0;
        mday = 0;
 
        scanf("%d %d %d", &gyear, &gmon, &gday);
        scanf("%d", &day);
 
        if (gyear < 0 || gmon <= 0 || gmon > 12 || gday <= 0 || gday > 31)
        {
                printf("Error -> Date error!!\n");
 
                return 0;
        }
 
        if ((gyear%4) == 0 && (gyear%100) != 0 || (gyear%400) == 0)
        {
                lastday[1] = 29;
        }
 
        else
        {
                lastday[1] = 28;
        }
 
        for (i=1; i<gmon; i++)
        {
                sumday += lastday[i-1];
        }
 
        jday = sumday + gday;
 
        pday = jday + day;
        mday = jday - day;
 
        printf ("%d\n", jday);
        printf ("%d\n", pday);
        printf ("%d\n", mday);
 
/*
        for (i=0; i<12; i++)
        {
                sumday += lastday[i];
 
                if (sumday >= jday && sumday-lastday[i] < jday)
                {
                        gmon = i+1;
                        gday = jday - sumday + lastday[i];
                }
        }
 
        printf ("%d %d %d", gyear, gmon, gday);
*/
 
        return 0;
}