[Javascript] 日期增加運算

常用的增加時間運算函式，有興趣的朋友可以選用。可以自行設定Interval，非常方便:D
Reference: http://stackoverflow.com/questions/1197928/how-to-add-30-minutes-to-a-javascript-date-object

 function dateAdd(date, interval, units) {
        var ret = new Date(date); //don't change original date
        switch(interval.toLowerCase()) {
            case 'year'   :
                ret.setFullYear(ret.getFullYear() + units);
                break;
            case 'quarter':
                ret.setMonth(ret.getMonth() + 3*units);
                break;
            case 'month'  :
                ret.setMonth(ret.getMonth() + units);
                break;
            case 'week'   :
                ret.setDate(ret.getDate() + 7*units);
                break;
            case 'day'    :
                ret.setDate(ret.getDate() + units);
                break;
            case 'hour'   :
                ret.setTime(ret.getTime() + units*3600000);
                break;
            case 'minute' :
                ret.setTime(ret.getTime() + units*60000);
                break;
            case 'second' :
                ret.setTime(ret.getTime() + units*1000);
                break;
            default       :
                ret = undefined;
                break;
        }
        return ret;
    }