window.onload = function() {
/* set your parameters(
number to countdown from,
pause between counts in milliseconds,
function to execute when finished
)
*/
startCountDown(30, 1000);
}

function startCountDown(i, p) {
// store parameters
var pause = p;
// make reference to div
var countDownObj = document.getElementById("cd");
if (countDownObj == null) {
// error
alert("div not found, check your id");
// bail
return;
}
countDownObj.count = function(i) {
// write out count
countDownObj.innerHTML = i;
if (i == 0) {
// stop
return;
}
setTimeout(function() {
// repeat
countDownObj.count(i - 1);
},
pause
);
}
// set it going
countDownObj.count(i);
}