// In a real program one would probably not use time()
  // Also, some older pre-release versions of the realtime
  // kernel patches had the clock_nanosleep() and time() 
  // clocks out of sync by 0.5s (IIRC)
  for(;;) {
    struct timespec after_epoch = { .tv_sec = time(NULL) + 1, .tv_nsec = 0 };
    // Check for EINTR in a real program
    clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &after_epoch, NULL);
    printf("Tick\n");
  }

Unfortunately, this probably doesn't help with Flash or JavaScript clock demos.

Alternatively, use time() to only sleep the remaining timespan until the next second.