#include <time.h>
#include <stdio.h>

#define NANO 10000000000


int multiples_three_five(int limit) 
{
    long total = 0;

    for (int i=1; i < limit; i++)
        total += i % 3 == 0 || i % 5 == 0 ? i : 0;

    return total;
}

int main(void) 
{
    struct timespec start;
    clock_gettime(CLOCK_REALTIME, &start);
    double start_time = ((float) start.tv_sec) + ((float) start.tv_nsec) / NANO; 

    long ans = multiples_three_five(1000);
    printf ("Answer: %ld\n", ans); 

    struct timespec stop;
    clock_gettime(CLOCK_REALTIME, &stop);
    double stop_time = ((float) stop.tv_sec) + ((float) stop.tv_nsec) / NANO;

    printf ("Time: %.8f\n", stop_time - start_time);

    return 0;
}
    

#include <stdio.h>
#include <time.h>


int multiples_three_five(int limit) 
{
    long total = 0;

    int i;
    for (i=1; i < limit; i++)
        total += i % 3 == 0 || i % 5 == 0 ? i : 0;
    return total;
}

int main(void) 
{
    clock_t start, stop;

    start = clock();
    long ans = multiples_three_five(1000);

    printf ("Answer: %ld\n", ans); 

    stop = clock(); 
    printf ("Time: %f\n", ((float)stop - (float)start) / CLOCKS_PER_SEC);

    return 0;
}
    

--- 
+++ 
@@ -1,28 +1,33 @@
+#include <time.h>
 #include <stdio.h>
-#include <time.h>
+
+#define NANO 10000000000
 
 
 int multiples_three_five(int limit) 
 {
     long total = 0;
 
-    int i;
-    for (i=1; i < limit; i++)
+    for (int i=1; i < limit; i++)
         total += i % 3 == 0 || i % 5 == 0 ? i : 0;
+
     return total;
 }
 
 int main(void) 
 {
-    clock_t start, stop;
+    struct timespec start;
+    clock_gettime(CLOCK_REALTIME, &start);
+    double start_time = ((float) start.tv_sec) + ((float) start.tv_nsec) / NANO; 
 
-    start = clock();
     long ans = multiples_three_five(1000);
-
     printf ("Answer: %ld\n", ans); 
 
-    stop = clock(); 
-    printf ("Time: %f\n", ((float)stop - (float)start) / CLOCKS_PER_SEC);
+    struct timespec stop;
+    clock_gettime(CLOCK_REALTIME, &stop);
+    double stop_time = ((float) stop.tv_sec) + ((float) stop.tv_nsec) / NANO;
+
+    printf ("Time: %.8f\n", stop_time - start_time);
 
     return 0;
 }
Use WALL time for timer
