How do I put a live time(seconds) counter on a page?

function timer(seconds){
    clearInterval(countdown);
    displayRemainingTime(seconds);
    const then = Date.now() + seconds * 1000;
    countdown = setInterval(() => {
        const secondsLeft = Math.round((then - Date.now()) / 1000);
        if(secondsLeft < 0) {
            clearInterval(countdown);
            return;
        }
        displayRemainingTime(secondsLeft);
    }, 1000)
}

function displayTimeLeft(){
 //update dom
}