document.write("<a href='http://www.adconnection.cn/trackingcode.php?aid=3&linkid=B31' target='_blank' onclick='return getCookie();'><img src='http://www.sephora.cn/promotion/cps200808futurebeauty/185-230.gif'  border='0' width='185' height='230' ></a>");<!-- Jascript cookie checkking -->
function getCookie() {
    var name  = "myCookie";
	var start = document.cookie.indexOf( name + "=" );
	var abc=document.cookie;
	var len   = start + name.length + 1;
	if ( (start<0) && 	( name != document.cookie.substring( 0, name.length ) ) )
	{
		setCookie();
    	return true;
	}
	if ( start == -1 ) {
        setCookie();
    	return true;
    }
	var end = document.cookie.indexOf( ";", len );
	if ( end == -1 ) end = document.cookie.length;
    var cookieTime = document.cookie.substring( len, end );
    var varTime   = new Date();
    var currTime  = varTime.getTime();
    if(cookieTime>currTime) {
         return false;
    }
    else {
        setCookie();
        return true;
    }
 }
<!-- set cookie time -->

function setCookie() {
  var expdate 	  = new Date ();
  expdate.setTime(expdate.getTime()+20*1000);
  document.cookie = "myCookie = " + expdate.getTime() + " ; expires = " + expdate;
}