The problem you are having is that you are using window.onload and not the iframe's onload attribute. The input button triggered both Reload() and pageloadingtime() which is also a big no no. I fixed your code by moving the setting of beforeload also into Reload() and attaching the trigger to iframe's onload attribute:
<html>
<head>
<script type="text/javascript">
//calculate the time before calling the function in window.onload
beforeload = (new Date()).getTime();
function pageloadingtime()
{
//calculate the current time in afterload
afterload = (new Date()).getTime();
// now use the beforeload and afterload to calculate the seconds
secondes = (afterload-beforeload)/1000;
document.getElementById("loadingtime").innerHTML = "<font color='red'>(You Page Load took " + secondes + " seconde(s).)</font>";
}
//window.onload = pageloadingtime;
</script>
<script type="text/javascript">
function Reload() {
beforeload = (new Date()).getTime();
var f = document.getElementById('iframe1');
f.src = f.src;
}
</script>
<title>load time</title>
</head>
<body>
<div id="loadingtime"></div>
<br>
<iframe id="iframe1" width="800" height="350" src="http://google.com" onload="pageloadingtime();"></iframe>
<br>
<input type="button" value="Reload" onClick="Reload();">
<br>
</body>
</html>
Have fun!
-MRB