【最简单的时钟】【setInterval】

    Date.prototype.format = function(format){
        var o = {
            "M+" : this.getMonth()+1, //month
            "d+" : this.getDate(), //day
            "h+" : this.getHours(), //hour
            "m+" : this.getMinutes(), //minute
            "s+" : this.getSeconds(), //second
            "q+" : Math.floor((this.getMonth()+3)/3), //quarter
            "S" : this.getMilliseconds() //millisecond
        }

        if(/(y+)/i.test(format)) {
            format = format.replace(RegExp.$1, (this.getFullYear()+"").substr(4 - RegExp.$1.length));
        }

        for(var k in o) {
            if(new RegExp("("+ k +")").test(format)) {
                format = format.replace(RegExp.$1, RegExp.$1.length==1 ? o[k] : ("00"+ o[k]).substr((""+ o[k]).length));
            }
        }
        return format;
    }
    function setTime() {
        var mydate = new Date();
//        var t = mydate.toLocaleString();
        var t = mydate.format("yyyy-MM-dd hh:mm:ss");;
        $('#dt').text(t);
    }

    setInterval("setTime()","1000");// 一秒运行一次，获取当前系统时间