How to get the week in a month for a date with Javascript

Working on my calendar application and I needed to get what week in the current month a day occurs on. Unfortunately I couldn’t find anything like this in the JavaScript Date object so this is the function I came up with

Date.prototype.getMonthWeek = function(){
	var firstDay = new Date(this.getFullYear(), this.getMonth(), 1).getDay();
	return Math.ceil((this.getDate() + firstDay)/7);
}

Thanks to mutilator for helping out with the actual math to figure it out.