alert(getDate());            //當前時間
alert(getDate("day", -2));   //前兩天的時間
alert(getDate("day", 2));    //後兩天的時間
alert(getDate("month", -2)); //前兩個月的時間
alert(getDate("month", 2));  //後兩個月的時間

function getDate(type=null,number=0) {
    var nowdate = new Date();
    switch (type) {
        case "day":   //取number天前、後的時間
            nowdate.setTime(nowdate.getTime() + (24 * 3600 * 1000) * number);
            var y = nowdate.getFullYear();
            var m = nowdate.getMonth() + 1;
            var d = nowdate.getDate();
            var retrundate = y + '/' + m + '/' + d;
            break;
        case "week":  //取number周前、後的時間
            var weekdate = new Date(nowdate + (7 * 24 * 3600 * 1000) * number);
            var y = weekdate.getFullYear();
            var m = weekdate.getMonth() + 1;
            var d = weekdate.getDate();
            var retrundate = y + '/' + m + '/' + d;
            break;
        case "month":  //取number月前、後的時間
            nowdate.setMonth(nowdate.getMonth() + number);
            var y = nowdate.getFullYear();
            var m = nowdate.getMonth() + 1;
            var d = nowdate.getDate();
            var retrundate = y + '/' + m + '/' + d;
            break;
        case "year":  //取number年前、後的時間
            nowdate.setFullYear(nowdate.getFullYear() + number);
            var y = nowdate.getFullYear();
            var m = nowdate.getMonth() + 1;
            var d = nowdate.getDate();
            var retrundate = y + '/' + m + '/' + d;
            break;
        default:     //取當前時間
            var y = nowdate.getFullYear();
            var m = nowdate.getMonth() + 1;
            var d = nowdate.getDate();
            var retrundate = y + '/' + m + '/' + d;
    }
    return retrundate;
}