JS 时间向前或向后推迟天数

function getthedate(dd, dadd) {
var a = new Date(dd)
a = a.valueOf()
a = a - dadd * 24 * 60 * 60 * 1000
a = new Date(a);
var m = a.getMonth() + 1;
if (m.toString().length == 1) {
m = '0' + m;
}
var d = a.getDate();
if (d.toString().length == 1) {
d = '0' + d;
}
return a.getFullYear() + "/" + m + "/" + d;
}