JS 转换Excel日期数字 转回日期格式

//excel日期格式转换 numb为数字，format为拼接符“-”
    function formatDate(numb, format) {
        if (numb != undefined) {

            let time = new Date((numb - 1) * 24 * 3600000 + 1)
            (() - 70)
            let year = () + ''
            let month = () + 1 + ''
            let date = () + ''
            if (format &&  === 1) {
                return year + format + month + format + date
            }
            return year + (month < 10 ? '0' + month : month) + (date < 10 ? '0' + date : date)
        } else {
            return undefined;
        }
    }