how to display last visited user's date and time in header of the webpage??
You'll probably need to sniff it out with PHP, save that data to a database, then echo that out with a PHP script. HTML won't do it alone.
Try the following code:
<head><title>System Date and Time</title></head>
var date = new Date();
document.write("<b>System date and Time is:</b> "+ date);
All that's going to do is display the current date in a linux format. It doesn't even begin to address what was asked here.