//beginDate 需要参加计算的日期，days是要减去的天数，返回新的日期，日期格式：YYYY-MM-DD
function getDay(beginDate, days) {
    var beginDate = beginDate.split("-");
    var nDate = new Date(beginDate[1] + '-' + beginDate[2] + '-' + beginDate[0]); //转换为MM-DD-YYYY格式  
    var millSeconds = Math.abs(nDate) - (days * 24 * 60 * 60 * 1000);
    var rDate = new Date(millSeconds);
    var year = rDate.getFullYear();
    var month = rDate.getMonth() + 1;
    if (month < 10) month = "0" + month;
    var date = rDate.getDate();
    if (date < 10) date = "0" + date;
    return (year + "-" + month + "-" + date);
}

var res = getDay('2021-10-27',90)
console.log(res);