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> <script type="text/javascript"><!-- google_ad_client = "pub-1985112462197986"; /* 728x90, 创建于 10-7-27 */ google_ad_slot = "9407691718"; google_ad_width = 728; google_ad_height = 90; //--> </script><script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js"> </script>