//計算日期加法
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(d.getDate() + number);
        return date;
        break;
    }
    }
}

//把String類的日期轉換為Date類

var date = new Date(2017-09-14 00:00:00.000);

//然後計算

var nextDate = DateAdd("y",3,date)

js把String類轉換為日期類並計算