js怎么让时间函数的秒数在页面上显示是变化的

<input type="text" id="showtime" value="" />
<script type="text/javascript">
function ymdhis(){
    var obj = new Date();
    var y = obj.getFullYear();
    var m = obj.getMonth()+1;
    var d = obj.getDate();
    var h = obj.getHours();
    var i = obj.getMinutes();
    var s = obj.getSeconds();
    if(m<10)m="0"+m;
    if(d<10)d="0"+d;
    if(h<10)h="0"+h;
    if(i<10)i="0"+i;
    if(s<10)s="0"+s;
    var time = y+'-'+m+'-'+d+' '+h+':'+i+':'+s;
    document.getElementById("showtime").value=time;
    setTimeout(ymdhis,1000);
}
ymdhis();
</script>