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

int main(void)
{
   time_t timer;
   struct tm *t_st;
   char systemDate[8+1];

   int Birthday=19801020;
   int Nenrei=0;

   /* 現在時刻の取得 */
   time(&timer);

   /* 現在時刻を構造体に変換 */
   t_st = localtime(&timer);

   /* yyyymmddの形式に編集 */
   sprintf(systemDate,"%4d%02d%02d",t_st->tm_year+1900,t_st->tm_mon+1,t_st->tm_mday);

   Nenrei = (int)systemDate - Birthday;

   printf("Nenrei=%d\n",Nenrei);

   return 0;
}

>   char systemDate[8+1];
       ：
>   sprintf(systemDate,"%4d%02d%02d",t_st->tm_year+1900,t_st->tm_mon+1,t_st->tm_mday);
>
>   Nenrei = (int)systemDate - Birthday;

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

int main(void)
{
   time_t timer;
   struct tm *t_st;
   char systemDate[8+1];
   char Birthday[8+1]="19750316";
   int Nenrei=0;
   char myBirthOfThisYear[8+1];
   char gappi[4+1];
   char nen[4+1];

   /* 現在時刻の取得 */
   time(&timer);

   /* 現在時刻を構造体に変換 */
   t_st = localtime(&timer);

   /* yyyymmddの形式に編集 */
   sprintf(systemDate,"%4d%02d%02d",t_st->tm_year+1900,t_st->tm_mon+1,t_st->tm_mday);

   /* 誕生日の月日を取得 */
   memcpy(gappi,Birthday+4,4);
   gappi[4]='\0';

   /* 誕生日の年を取得 */
   memcpy(nen,Birthday,4);
   nen[4]='\0';

   /* 今年の自分の誕生日を求める */
   sprintf(myBirthOfThisYear,"%d%s",t_st->tm_year+1900,gappi);
   printf("%s\n",myBirthOfThisYear);

   /* 今年の自分の誕生日をまだ迎えていないとき*/
   if(atol(systemDate) < atol(myBirthOfThisYear)){
        Nenrei = (t_st->tm_year+1900) - atoi(nen) - 1;
   }
   /* 今年の誕生日をすでに迎えているとき */
   else{
        Nenrei = (t_st->tm_year+1900) - atoi(nen);
   }

   printf("%d\n",Nenrei);

   return 0;
}

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

int main(void)
{
    time_t now    = time(NULL);
    struct tm *tp = localtime(&now);
    int birthday  = 19801020;
    int year  = birthday / 10000;
    int month = birthday % 10000 / 100;
    int day   = birthday / 100;

    printf("%d\n", (tp->tm_year + 1900 - year)
                 - (tp->tm_mon + 1 < month ||
                    tp->tm_mon + 1 == month && tp->tm_mday < day));
    return 0;
}

    char birthday[] = "19801020";
    int year, month, day;
    sscanf(birthday, "%4d%2d%2d", &year, &month, &day);