也写dateUtil.js

yl.dateUtil = {
    /**
     *  y   年
     *  M   月
     *  d   日
     *  H   时            h   时（am/pm）
     *  m   分
     *  s   秒
     *  S   毫秒
     *  a   上午/下午（am/pm)
     *  setInterval(function(){
     *     input111.value = yl.dateUtil.dateStr(new Date(), "要一个给小于9的d数m字s前H面M加0返回字符串的方法yyyy");
     *  }, 1000);
     */
    dateStr: function (date, str) {
        if (!date instanceof Date) {
            return str;
        }
        //match之后得到数组
        //遍历数组，
        //判断各个元素长度、内容，做对应替换
        var reg = /yyyy|yy|MM|M|dd|d|HH|H|h|mm|m|ss|s|SSS|S|a/g;
        var strArr = str.match(reg);
        for (var i = 0, len = strArr.length; i < len; i++) {
            //console.log(strArr[i]);
            switch (strArr[i]) {
                case "yyyy":
                    str = str.replace(/yyyy/, date.getFullYear());
                    break;
                case "yy":
                    str = str.replace(/yy/, (date.getFullYear()+"").substr(2,2));
                    break;
                case "MM":
                    str = str.replace(/MM/, this.getLe9Str(date.getMonth()+1));
                    break;
                case "M":
                    str = str.replace(/M/, date.getMonth()+1);
                    break;
                case "dd":
                    str = str.replace(/dd/, this.getLe9Str(date.getDate()));
                    break;
                case "d":
                    str = str.replace(/d/, date.getDate());
                    break;
                case "HH":
                    str = str.replace(/HH/, this.getLe9Str(date.getHours()));
                    break;
                case "H":
                    str = str.replace(/H/, date.getHours());
                    break;
                case "h":
                    var s = date.getHours();
                    str = str.replace(/h/, s > 12 ? s % 12 : s);
                    break;
                case "mm":
                    str = str.replace(/mm/, this.getLe9Str(date.getMinutes()));
                    break;
                case "m":
                    str = str.replace(/m/, date.getMinutes());
                    break;
                case "ss":
                    str = str.replace(/ss/, this.getLe9Str(date.getSeconds()));
                    break;
                case "s":
                    str = str.replace(/s/, date.getSeconds());
                    break;
                case "SSS":
                    str = str.replace(/SSS/, this.getLe99Str(date.getMilliseconds()));
                    break;
                case "S":
                    str = str.replace(/S/, date.getMilliseconds());
                    break;
                case "a":
                    str = str.replace(/a/, date.getHours() > 12 ? "pm" : "am");
                    break;
            }
        }
        //console.log(str);
        return str;
    },
    strDate: function () {

    },
    //要一个给小于9的数字前面加0返回字符串的方法
    getLe9Str: function (num) {
        return num > 9 ? "" + num : "0" + num;
    },
    //小于100前面加00或者0
    getLe99Str: function (num) {
        if (num > 9 && num < 100) {
            return "0"+num;
        } else {
            return this.getLe9Str(num);
        }
    }
};

调用

<body>
    <input type="text" id="input111" style="500px;"/>
<script type="text/javascript" src="ylUtil.js"></script>

<script type="text/javascript">

var input111 = document.getElementById("input111");
setInterval(function(){
    input111.value = yl.dateUtil.dateStr(new Date(), "a h:mm:ss`SSS~~~~yyyy年M月d日");
    }, 1);
</script>
</body>

ps：

觉得js用new Date("2013/08/01 11:20:00")的方式还是挺好的。所以strDate没写了。
dateStr方法用到参数格式从java的simpleDateFormat里面取的。所以与它的是一致的。
用来把个日期转成指定字符串个人觉得够用了，关于性能问题，欢迎各路大虾提点。
http://www.cnblogs.com/nimeiz/p/3446292.html