use var!!

cron-test.js

var cron = require('./cron'), sys = require('sys');
cron.Every((2).seconds(), function() { sys.puts('Working!'); });

cron.js

var sys = require('sys');
function TimeInterval() {
	this.seconds = 0;
	this.addTo = function(ti) {
		this.seconds += ti.seconds;
		return this;
	}
}

var interv = new TimeInterval();
if(interv.seconds == 0)
{
	sys.puts('Success!');
}

var fsinterv = new TimeInterval();
fsinterv.seconds = 5;
if(interv.addTo(fsinterv).seconds == 5)
{
	sys.puts('Success!');
}

function generate_multiplier(multiplier)
{
	return function() {
		var interval = new TimeInterval(); // use var keyword!
		interval.seconds = this * multiplier;
		return interval;
	}
}

Number.prototype.seconds = generate_multiplier(1);
Number.prototype.minutes = generate_multiplier(60); 
Number.prototype.hours = generate_multiplier(3600);

if((5).seconds().seconds == 5)
{
	sys.puts('Success!');
}

if((5).minutes().seconds == 300)
{
	sys.puts('Success!');
}

if((5).hours().seconds == 18000)
{
	sys.puts('Success!');
}

if((5).minutes().addTo((5).minutes()).seconds == 600)
{
	sys.puts('Success!');
}

exports.Every = function(timeInterval, callback) {
	setInterval(callback, timeInterval.seconds * 1000);
}

// Test
//exports.Every((5).seconds(), function () { sys.puts("Hi!") });