2048 speed run timer

var start = document.getElementsByClassName('restart-button');
function startTimer() {
     window.gameStart = +new Date();
}
startTimer();
for (var i=0;i<start.length;i++) {
    start[i].onclick = startTimer;
}
function pad0(n) {return (n<10?'0':'')+n;}
function toHMS(t) {
t=Math.floor((+new Date() - t)/1000);
    return (Math.floor((t-(t%3600))/3600))+':'+pad0(Math.floor((t%3600)/60))+':'+pad0(t%60);
}
twttr.widgets.load = function(won) {
    var curScore = document.getElementsByClassName('score-container')[0].childNodes[0].textContent;
    document.getElementsByClassName('score-sharing')[0].innerHTML = curScore + ' points in ' + toHMS(gameStart) + ' (' + Math.floor(+curScore / ((+new Date() - gameStart)/1000)) + ' / sec)';
    startTimer();
}