std::difftime

Giriş
Farkı saniye ve double cinsinden döner.

Örnek
Şöyle yaparız.

time_t start = time(NULL);
...
time_t now = time(NULL);

double diff = difftime(now, start);

unsigned long long seconds = diff;
unsigned long long h = seconds / 3600;
seconds %= 3600;
unsigned long long m = seconds / 60;
seconds %= 60;
unsigned long long s = seconds;

printf("\r%02llu:%02llu:%02llu", h, m, s);