corrected cron.js

cron-test.js

// Sample for http://oliveriskindoffunny.tumblr.com/post/247975858/implementing-a-user-friendly-cron-module-with-node-js
var cron = require('./cron'), sys = require('sys');
cron.Every((2).seconds(), function() { sys.puts('Working!'); });

cron.js

// Sample for http://oliveriskindoffunny.tumblr.com/post/247975858/implementing-a-user-friendly-cron-module-with-node-js
function TimeInterval() {
	this.seconds = 0;
	this.addTo = function(ti) {
		this.seconds += ti.seconds;
		return this;
	};
}

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);

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