参数:interval,字符串表达式，表示要添加的时间间隔.
参数:number,数值表达式，表示要添加的时间间隔的个数.
参数:date,时间对象.
返回:新的时间对象.

function DateAdd(interval, number, date) {
    switch (interval) {
        case "y ": {
            date.setFullYear(date.getFullYear() + number);
            return date;
            break;
        }
        case "q ": {
            date.setMonth(date.getMonth() + number * 3);
            return date;
            break;
        }
        case "m ": {
            date.setMonth(date.getMonth() + number);
            return date;
            break;
        }
        case "w ": {
            date.setDate(date.getDate() + number * 7);
            return date;
            break;
        }
        case "d ": {
            date.setDate(date.getDate() + number);
            return date;
            break;
        }
        case "h ": {
            date.setHours(date.getHours() + number);
            return date;
            break;
        }
        case "m ": {
            date.setMinutes(date.getMinutes() + number);
            return date;
            break;
        }
        case "s ": {
            date.setSeconds(date.getSeconds() + number);
            return date;
            break;
        }
        default: {
            date.setDate(date.getDate() + number);
            return date;
            break;
        }
    }
}

var now = new Date();
// 加五天.
var newDate = DateAdd("d ", 5, now);
alert(newDate.toLocaleDateString())
// 加两个月.
newDate = DateAdd("m ", 2, now);
alert(newDate.toLocaleDateString())
// 加一年
newDate = DateAdd("y ", 1, now);
alert(newDate.toLocaleDateString())