setInterval()
is a method that can be used to execute JavaScript script at specified intervals. We use it to display time from Date() class.
HTML
<div id="time"></div>

JavaScript
<script type="text/javascript"> var time = document.getElementById("time"); function showTime() { var date = new Date().toLocaleString("en-US", {timeZone: "Asia/Singapore"}); time.innerHTML = date; } setInterval(showTime, 1000); </script>
With custom format
<script type="text/javascript"> var time = document.getElementById("time"); function showTime() { var d = new Date(); time.innerHTML = d.getFullYear() + '-' + d.getMonth() + '-' + d.getDate() + ' ' + d.getHours() + ":" + d.getMinutes() + ":" + d.getSeconds(); } setInterval(showTime, 1000); </script>