// Week number (ISO 8601)
Date.prototype.getWeek = function () {
	// Copies this date
	var s = new Date(+this);
	// Set day to the first thursday of the year
	s.setDate(s.getDate() + 4 - (s.getDay() || 7));
	// return the weeknumber of this date
	return ~~(~~((s.getTime() - new Date(s.getFullYear(), 0, 1)) / 8.64e7) / 7) + 1;
}