JavaScript获取过去几天或者将来几天的日期

function getLastOrNextFewDateBy(day){
     var today = new Date();
     var lastOrNextDate = today.getTime() + 1000 * 60 * 60 * 24 * day;
     today.setTime(lastOrNextDate);
     var year = today.getFullYear();
     var month = today.getMonth() + 1;
     var date = today.getDate();
     month = month < 10 ? '0' + month : month;
     date = date < 10 ? '0' + date : date;
     return year + "-" + month + "-" + date;
}
console.log(getLastOrNextFewDateBy(-7));//过去七天        2018-05-14
console.log(getLastOrNextFewDateBy(0)); //今天           2018-05-21
console.log(getLastOrNextFewDateBy(7)); // 将来七天       2018-05-28