JS计算几天后的日期

function AddDays(num) {
var newdate = new Date();
var newtimems = newdate.getTime() - (num * 24 * 60 * 60 * 1000);
newdate.setTime(newtimems);
var time = newdate.getFullYear() + "-" + (newdate.getMonth()+1) + "-" + newdate.getDate();
return time;
}