Adding/Subtracting Business Days in Javascript (extends the native Date Javascript object)

businessDays.js

Number.prototype.mod = function(n) {
  return ((this%n)+n)%n;
}

// support for adding/subtracting business days for Javascript dates
Date.prototype.addBusinessDays = function(days) {
	days = parseInt(days);
	var wks = Math.floor(days/5);
	var dys = days.mod(5);
	var dy = this.getDay();

	if (dy === 6 && dys > -1) {
	   if (dys === 0) {dys-=2; dy+=2;}
	   dys++; dy -= 6;}
	if (dy === 0 && dys < 1) {
	   if (dys === 0) {dys+=2; dy-=2;}
	   dys--; dy += 6;}
	if (dy + dys > 5) dys += 2;
	if (dy + dys < 1) dys -= 2;
	this.setDate(this.getDate()+wks*7+dys);

	return this;
}