computing time elapsed in C?

/**
 * this function is for computing the time difference between timeval x and y
 * the result is stored in result
 */
int
timeval_subtract (struct timeval *result, struct timeval *x, struct timeval *y)
{
    /* Perform the carry for the later subtraction by updating y. */
    if (x->tv_usec < y->tv_usec) {
        int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
        y->tv_usec -= 1000000 * nsec;
        y->tv_sec += nsec;
    }
    if (x->tv_usec - y->tv_usec > 1000000) {
        int nsec = (x->tv_usec - y->tv_usec) / 1000000;
        y->tv_usec += 1000000 * nsec;
        y->tv_sec -= nsec;
    }

    /* Compute the time remaining to wait.
    tv_usec is certainly positive. */
    result->tv_sec = x->tv_sec - y->tv_sec;
    result->tv_usec = x->tv_usec - y->tv_usec;

    /* Return 1 if result is negative. */
    return x->tv_sec < y->tv_sec;
}

int main(int argc, char *argv[])
{
    struct timeval start, stop, echodelay;  // start, stop and echo delay times

    if((gettimeofday(&start, NULL)) == -1)
    {
        perror("gettimeofday");
        exit(1);
    }


    // do stuff


    if((gettimeofday(&stop, NULL)) == -1)
    {
        perror("gettimeofday");
        exit(1);
    }

    /* compute time delay */
    timeval_subtract(&echodelay, &stop, &start);

    printf("Echo delay is %ds and %dus\n", echodelay.tv_sec, echodelay.tv_usec);
}

#include <time.h>

int main(int argc, char *argv[]) {
   time_t start, stop;
   double diff

   start = time(NULL);
   // Do stuff

   stop = time(NULL);
   diff = difftime(start, stop);

   printf("Difference is %e\n", diff);
   return 0;
}