Temporizador React

index.html

<div id="root"></div>

script.js

const Clock = props => {
 const [date, setDate] = React.useState(new Date());

 React.useEffect(() => {
  var timerID = setInterval( () => tick(), 1000 );
  return function cleanup() { clearInterval(timerID) }
 });

  function tick() {
    setDate(new Date());
  }

  return (
    <div>
      <h2>Hora actual {date.toLocaleTimeString()}</h2>
    </div>
  );
}

ReactDOM.render(
  <Clock />,
  document.getElementById('root')
);