You can simply create a small javascript that will return the time stamp of your user's computer. In the CSS/Javascript tab (Javascript area), just use something like this:

<script type="text/javascript">
function utime()
{
today = new Date();
document.getElmentById('name_of_hidden_field').value = today;
}
</script>


In the Additional attributes of the submit button just add something like this:

onclick="utime();"