根据指定的一个日期和相差的天数，获取另外一个日期,dateParameter为指定已经存在的日期yyyy-MM-dd num为相差天数为整型

addByTransDate.js

function addByTransDate(dateParameter, num) {

    var translateDate = "",
        dateString = "",
        monthString = "",
        dayString = "";
    translateDate = dateParameter.replace("-", "/").replace("-", "/");;

    var newDate = new Date(translateDate);
    newDate = newDate.valueOf();
    newDate = newDate + num * 24 * 60 * 60 * 1000; //备注 如果是往前计算日期则为减号 否则为加号
    newDate = new Date(newDate);

    //如果月份长度少于2，则前加 0 补位   
    if ((newDate.getMonth() + 1).toString().length == 1) {
        monthString = 0 + "" + (newDate.getMonth() + 1).toString();
    } else {
        monthString = (newDate.getMonth() + 1).toString();
    }

    //如果天数长度少于2，则前加 0 补位   
    if (newDate.getDate().toString().length == 1) {

        dayString = 0 + "" + newDate.getDate().toString();
    } else {

        dayString = newDate.getDate().toString();
    }

    dateString = newDate.getFullYear() + "-" + monthString + "-" + dayString;
    return dateString;

}