The JavaScript Date object can be used to count down to a specified date and time. For instance, this can be used as a prompt for the user to take action prior to a certain date.

<!-- timer will appear here -->
<h3>Only <span id="timer" style="color: red;"></span> remaining!</h3>

The following is set way into the future for the sake of creating a long-term example. But in reality, it might be set to expire in a week at midnight.

// set the date to count down to
let countDownDate = new Date('Jan 1, 2035 0:0:0').getTime();

// update the count down every second
let counter = setInterval(function() {

    // current date object
    let now = new Date().getTime();

    // difference between now and count down date
    let difference = countDownDate - now;

    // calculate days, hours, minutes and seconds
    let days = Math.floor(difference / (1000 * 60 * 60 * 24));
    let hours = Math.floor((difference % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
    let minutes = Math.floor((difference % (1000 * 60 * 60)) / (1000 * 60));
    let seconds = Math.floor((difference % (1000 * 60)) / 1000);

    // output time remaining
    document.getElementById('timer').innerHTML = days + 'd ' + hours + 'h ' + minutes + 'm ' + seconds + 's';

    // display a message when the count down expires
    if (difference < 1) {
        clearInterval(counter);
        document.getElementsByTagName('h3')[0].innerHTML = 'This offer has expired.';
    }
}, 1000);

Example:

Only 3d 11h 4m 13s remaining!