setInterval() 方法可按照指定的周期（以毫秒计）来调用函数或计算表达式

 <script type="text/javascript">
             setInterval(function(){
                 var myDate = new Date();
                 var $_year = myDate.getFullYear();
                 var $_month = parseInt(myDate.getMonth())+1;
                 var $_day = myDate.getDate();

                 var $_hour = myDate.getHours();
                 var $_minute = myDate.getMinutes();
                 var $_second = myDate.getSeconds();

                 if ($_hour < 10) {
                     $_hour = "0" + $_hour;
                 }
                 if ($_minute < 10) {
                     $_minute = "0" + $_minute;
                 }
                 if ($_second < 10) {
                     $_second = "0" + $_second;
                 }

                 var $_f_date2 = $_year +"年"+$_month+"月"+$_day+"日"+" "+$_hour+":"+$_minute+":"+$_second;
                 time.innerHTML = $_f_date2;
             },1000)  //每过1秒运行一次
             </script>