here's a jQuery solution:
Since you didn't specify, this example will add 10% of the entered value exponentially to the total value. Ie, every 3 seconds it will take 10% of the total and add it to the toal. Then 3 seconds later it will find 10% of the new total and ad it to it and so on.
Should be pretty straight forward as I added comments to the JS by the areas that need to be changes to suit your needs.
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function() {
// begin
// below is the amount you want to add in percentages (decimal)
var basepercent = ".10";
//below is the speed in which you want to add in miliseconds
var speed = "3000"; //that's 3 seconds. 30 minutes would be 1800000
$("#start").click(function () {
var starttime = $("#time").val();
$("span.num:eq(1)").text(starttime);
var percentage = basepercent * $("span.num:eq(1)").text()
$("span.num:eq(0)").text(percentage);
setInterval(function(){
var percentage = basepercent * $("span.num:eq(1)").text()
$("span.num:eq(0)").text(percentage);
var total = 0;
$("span.num").each(function() {
total += parseFloat($(this).text()) || 0;
});
total = Math.round(total);
$("#counter span").text(total);
}, speed);
});
// end
});
</script>
</head>
<body>
<form action="" method="post">
<input type="text" name="time" id="time">
<a href="#" id="start">Start</a>
<div>Your multiplier: <span class="num"></span></div>
</form>
<div id="counter">Your counter: <span class="num"></span></div>
</body>
</html>
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