timer.js

var seconds = 10;
var minutes = 0;

function start(){
    var time = setInterval(countdown, 1000);
}

document.getElementById('time').innerHTML = minutes + ':' + seconds;

function countdown(){
    if(seconds > 0){
        seconds--;
        document.getElementById('time').innerHTML = minutes + ':' + seconds;
    }
    else if(second <= 0){
        clearInterval(time);
        return;
    }
}

function add(){
    seconds++;
    document.getElementById('time').innerHTML = minutes + ':' + seconds;
}