In:

void os::Posix::print_uptime_info(outputStream* st) {
  int bootsec = -1;
  time_t currsec = time(nullptr);
  struct utmpx* ent;
  setutxent();
  while ((ent = getutxent())) {
    if (!strcmp("system boot", ent->ut_line)) {
      bootsec = ent->ut_tv.tv_sec;
      break;
    }
  }

  if (bootsec != -1) {
    os::print_dhm(st, "OS uptime:", currsec-bootsec);
  }
}

we have:

      bootsec = ent->ut_tv.tv_sec;

where bootsec is an int, but tv_sec is long.