For example you can try to settle it, by following way:
#include <time.h>
#include <stdio.h>
int main()
{
time_t your_time = ....
struct tm your_time_tm = *localtime( &your_time);
struct tm then_tm = your_time_tm;
then_tm.tm_sec += 100;
mktime( &then_tm);
printf( "%s\n", asctime( &your_time_tm));
printf( "%s\n", asctime( &then_tm));
return 0;
}