Please change your code as follows:
<script>
var button = document.getElementById("reactionTester");
var start = document.getElementById("start");
function init() {
var startInterval/*in milliseconds*/ = Math.floor(Math.random()*30)*1000;
setTimeout(startTimer,startInterval);
}
function startTimer(){
document.write("hey");
}
</script>
See if that helps. Basically, the difference is references the 'startTimer' function instead of executing it.