вот так попробуй
var timer;
var wnd;
function showTime()
{ var now = new Date();
wnd.document.open();
wnd.document.write("<h1 align='center'>");
wnd.document.write (now.toLocaleTimeString());
wnd.document.write ("</h1>");
wnd.close();
}
function startClock()
{
if(!timer)
{
var top = (screen.height-340)/2;
var left = (screen.width-270)/2;
var params = "width=300,height=100,top="+top+",left="+left;
wnd = window.open('','myClockWindow',params)
showTime()
timer = window.setInterval(showTime, 1000)
}
}