mktime() 함수 질문입니다

#include <ctime>
#include <cstdio>
 
int main ()
{
  struct tm first_tm, second_tm;
  time_t ftime, stime;
 
  first_tm.tm_year = 2016;
  first_tm.tm_mon = 5;
  first_tm.tm_mday = 14;
  first_tm.tm_hour = 12;
  first_tm.tm_min = 2;
  first_tm.tm_sec = 3;
 
  second_tm.tm_year = 2016;
  second_tm.tm_mon = 5;
  second_tm.tm_mday = 14;
  second_tm.tm_hour = 12;
  second_tm.tm_min = 10;
  second_tm.tm_sec = 11;
 
  ftime = mktime(&first_tm);  /* 디버거로 확인결과 리턴시 올바른값을 리턴하지만 대입하면서 이상한값으로 변함 */
  stime = mktime(&second_tm);
 
  ftime -= 10; /* 10초전을 구하고 싶었슴 */
  stime += 20; /* 20초후를 구하고 싶었슴 */
 
  time_t result_fisrt_tm, result_second_tm;
  result_first_tm = localtime(&ftime);
  result_second_tm = loacaltime(&stime);
 
  return 0;
}