Clock.jsx

import { useState, useEffect } from 'react'
const Clock = props => {
  const [date, setDate] = useState(props.date)
  const tick = () => setDate(new Date())
  useEffect(() => {
    const timerID = setInterval(tick, 1000)
    return () => clearInterval(timerID)
  }, [])
  return (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {date.toLocaleTimeString()}.</h2>
    </div>
  )
}

const App = props => {
  return (
    <>
      <Clock date={new Date} />
    </>
  )
}

export default App