判断html修改日期

<script type="text/javascript">
function timer()
{
var l = (new Date()-Date.parse(document.lastModified))/1000/3600/24;
//alert(l);
if(l>0.2)
{
new Ajax().getData("MakeIndex.aspx","isAjax=true",function(obj){var a="";});
}
}
timer();
</script>