html
<span id="show"></span>
js
<script type="text/javascript">
window.onload = function() {
var show = document.getElementById("show");
setInterval(function() {
var time = new Date();
var m = time.getMonth() + 1;
var t = time.getFullYear() + "-" + m + "-" +
time.getDate() + " " + time.getHours() + ":" +
time.getMinutes() + ":" + time.getSeconds();
show.innerHTML = t;
}, 1000);
};
</script>