timer.ts

  const [start] = useState(Date.now());
  const [, setRandomNum] = useState(0);

  const diff = Date.now() - start;

  useEffect(() => {
    const current = diff % 1000;
    const time = 1000 - current;
    const timer = setTimeout(() => {
      setRandomNum(Math.random());
    }, time);
    return () => {
      clearTimeout(timer);
    };
  });

  const num = 5 - Math.floor(diff / 1000);