function getBeforeDate(days){
var now=new Date().getTime();
var ago=now-86400000*days;//一天的毫秒数为86400000
var agoData= new Date(ago);
var year = agoData.getFullYear();
var mon = agoData.getMonth() + 1;
var day = agoData.getDate();
mon=mon<10? '0'+mon:mon;
day=day<10? '0'+day:day;
var date=year+'-'+mon+'-'+day;
return date;
};
console.log(getBeforeDate(7));//昨天的日期