Javascript, countdown timer and display text

Question

I am making a countdown timer with JavaScript.

Here is my script.

var seconds_left = 10;
var interval = setInterval(function() {
    document.getElementB         


        

Answer 1

document.getElementById('timer_div').innerHTML = "You are Ready!";

Answer 2

var seconds_left = 10;
var interval = setInterval(function() {
    document.getElementById('timer_div').innerHTML = --seconds_left;

    if (seconds_left <= 0)
    {
        document.getElementById('timer_div').innerHTML = 'You are ready';
        clearInterval(interval);
    }
}, 1000);

Here is the Example

Answer 3

<script>
    TargetDate = "12/31/2020 5:00 AM";
    DisplayFormat = "%%D%% Days, %%H%% Hours, %%M%% Minutes, %%S%% Seconds.";
    FinishMessage = "It is finally here!";
</script>
<script src="//scripts.hashemian.com/js/countdown.js"></script>