I am trying to get this time to countdown and i've got that working. But when it goes below 10 seconds I want it to display 00:09 instead of 00:9. 

How can I achieve this?

```
var timerbtn = document.getElementById("timer");
var counter = 20;
var newElement = document.createElement("p");
newElement.innerHTML = "00:20";
var id;
timerbtn.parentNode.replaceChild(newElement, timerbtn);

    function startTimer() {
this.style.display = 'none';
id = setInterval(function () {
    counter--;
    if (counter < 0) {
        newElement.parentNode.replaceChild(timerbtn, newElement);
        clearInterval(id);
     } else {
        newElement.innerHTML = "00:" +counter.toString() + "";
    }
     }, 1000);
    };

 var clickbtn = document.getElementById("btn");
 clickbtn.onclick = startTimer;

```

[Demo](http://jsfiddle.net/q5cfjvhf/)