1026 running time (15 minutes)

#define CLK_TCK 100
#include<stdio.h>

int main ()
{
    int h,m,s;
    double c1,c2,c3;
    scanf("%lf %lf",&c1,&c2);
    c3=(c2-c1)/CLK_TCK;
    s=c3+0.5;
    
    h=s/3600;
    s=s%3600;
    m=s/60;
    s=s%60;
    if(h<10)
        printf("0");
    printf("%d:",h);
    if(m<10)
        printf("0");
    printf("%d:",m);
    if(s<10)
        printf("0");
    printf("%d\n",s);
    return 0;
}