JavaScript - añadir boton para empezar en cero segundos

<div id="number"></div>

<script>

if(localStorage.getItem("tiempo") == null){

	localStorage.setItem("tiempo", (new Date()-0));

}

var n = parseInt(((new Date()-0)-parseInt(localStorage.getItem("tiempo")))/1000);

var l = document.getElementById("number");

setInterval(function(){

  l.innerHTML = n;

  n++;

},1000);

</script>

localStorage.setItem("tiempo", (new Date()-0));

var n = parseInt(((new Date()-0)-parseInt(localStorage.getItem("tiempo")))/1000);