如何使用Javascript获取距今n天前的日期

代码如下:
 function Test(day) {
                var today = new Date();
                var beforMilliseconds = today.getTime() - 1000 * 3600 * 24 * day;
                var beforday = new Date();
                beforday.setTime(beforMilliseconds);
                var strYear = beforday.getFullYear();
                var strDay = beforday.getDate();
                var strMonth = beforday.getMonth() + 1;
                if (strMonth < 10) {
                    strMonth = "0" + strMonth;
                }
                var strYesterday = strYear + "-" + strMonth + "-" + strDay;
                alert(strYesterday);
            }