SOURCE

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