CountDown.js

var secondsRemaining;
var intervalHandle;

function resetPage(){
	document.getElementById("inputArea").style.display = "block";
}
function tick(){
	var timeDisplay = document.getElementById("time");
	var min = Math.floor(secondsRemaining/60);
	var sec = secondsRemaining - min*60;
	
	if (sec<10){
		sec = "0" + sec;
	}
	
	var message = min + ":"+ sec;
	timeDisplay.innerHTML = message;
	if (secondsRemaining === 0){
		alert("wake up");
		clearInterval(intervalHandle);
		resetPage();
	}
	secondsRemaining--;
	
	
}

function startCountdown(){
	var minutes = document.getElementById("minutes").value;
	if (isNaN(minutes)) {
		alert("please enter a number");
		return;
	}
	secondsRemaining = minutes*60;
	intervalHandle = setInterval(tick, 1000);
	document.getElementById("inputArea").style.display="none";
}

window.onload = function(){
	var inputMinutes = document.createElement("input");
	inputMinutes.setAttribute("id", "minutes");
	inputMinutes.setAttribute("type", "text");
	
	var startButton = document.createElement("input");
	startButton.setAttribute("type", "button");
	startButton.setAttribute("value", "start countdown");
	startButton.onclick = function(){
		startCountdown();
	};
	document.getElementById("inputArea").appendChild(inputMinutes);
	document.getElementById("inputArea").appendChild(startButton);

};