C语言怎么计算时间

输入一个时间，
输入时间间隔，
得到最终时间。
time_t TimeCString(const char *src)
{
int length = strlen(src);
if(14 != length)
{
printf("Error! src %s strlen(src) %d (%s %d %s)\n", src, strlen(src), __FILE__, __LINE__, __func__);
return -1;
}
struct tm tm;
memset(&tm, 0, sizeof(tm));
// 省略了&0XFF操作
tm.tm_year = ((0XFF&src[0])-'0')*1000+((0XFF&src[1])-'0')*100+((0XFF&src[2])-'0')*10+((0XFF&src[3])-'0')-1900;
tm.tm_mon = ((0XFF&src[4])-'0')*10+((0XFF&src[5])-'0');
tm.tm_mday = ((0XFF&src[6])-'0')*10+((0XFF&src[7])-'0');
tm.tm_hour = ((0XFF&src[8])-'0')*10+((0XFF&src[9])-'0');
tm.tm_min = ((0XFF&src[10])-'0')*10+((0XFF&src[11])-'0');
tm.tm_sec = ((0XFF&src[12])-'0')*10+((0XFF&src[13])-'0');
return mktime(&tm);
}
void CalcEndTime(char *end_time, const char *start_time, int duration)
{
time_t t_end_time = TimeCString(start_time) + duration;
struct tm *tm = localtime(&t_end_time);
sprintf(end_time, "%04d%02d%02d%02d%02d%02d", tm->tm_year+1900, tm->tm_mon, tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec);
}
没个月都有31天，？
why?
C语言的时间函数怎么用？