Patterns for deriving state gist4

code.js

function ticker(){
    const [renderTimes,rerender] = useState(0);
    const now = new Date().getTime();
    const [start] = useState(now);
    //we use this to force the function to rerender every second
    useEffect(()=>{
        const intervalId = setInterval(()=>rerender(renderTimes + 1),1000)
        return ()=>clearInterval(intervalId)
    })
    return <div>
        seconds elapsed {Math.floor((now - start)/1000}
    </div>
}