为什么clearInterval停止不了？

<div class="box">

<div id="box1">

<span id="share">分享</span>

</div>

</div>

window.onload=function(){

var oDiv=document.getElementById("box1");

oDiv.onmouseover = function(){

startMove();

}

}

var timer = null;

function startMove(){

var oDiv=document.getElementById("box1");

timer = setInterval(function(){

if(oDiv.offsetLeft == 0){

clearInterval(timer)

}else{

oDiv.style.left = oDiv.offsetLeft+1+'px';

}

},30)

}