A JavaScript module to extend the Date prototype to provide an easy method for adding to or subtracting from a Date instance in business days.

addBusinessDays.js

if(Number.prototype.mod === undefined)
{
	Number.prototype.mod = function(n) {
		return ((this%n)+n)%n;
	}
}
if(Date.prototype.addBusinessDays === undefined)
{
	Date.prototype.addBusinessDays = function(dd) {
		var weeks = (dd >= 0) ? Math.floor(dd/5): Math.ceil(dd/5);
		var days = dd % 5;
		var day = this.getDay();
		
		if (day === 6 && days > -1)
		{
		  if (days === 0)
		  {
		    days-=2;
			day+=2;
		  }
		  days++;
		  day -= 5;
		}
		
		if (day === 0 && days < 1)
		{
		  if (days === 0)
		  {
			days+=2;
			day-=2;
		  }
		  days--;
		  day += 5;
		}
		
		if (day + days > 5){days += 2};
		if (day + days < 1){days -= 2};
		
		this.setDate(this.getDate()+weeks*7+days);
	}
}

My primary contribution to this piece of history from http://javascript.about.com/library/blbusdayadd.htm can be found on line 10. The original implementation just did a Math.floor() on dd/5 which works great until you're subtracting business days and now all of a sudden the "next business day" is 1 day past your target. floor() behaves differently for negative numbers. For this we want more of a ceil() effect in that situation. Oddly enough, VBScript's Fix() method behaves as JS's ceil() for negative numbers and like JS's floor() for positive.