js页面停留时间

//在线阅读时间
function onlineRead(minutes){
        var o= new Object();
        o.courseId=courseId;
        o.userId=userId;
        o.onlineTime=minutes;
        var json=JSON.stringify(o);
        if(countryId!='999'){//非总部用户的在线阅读时长
            $.ajax({
                url:"/topsale/common/saveData.action?sqlName=train.insertOnline&paging=-1",
                data:{data:json},
                type:"POST",
            });
        }    
}

var start;
    var end;
    var duration=0;
    
    start= new Date();
    window.onbeforeunload = function(e) {
        end = new Date();
        duration=end.getMinutes()-start.getMinutes();
        //计算出小时数
//        var minute =Math.round(duration/(1000*60));//取的是分
        onlineRead(duration);
    };