AddBusinessDays

/* Extension */
public static DateTime AddBusinessDays(this DateTime current, int days, IEnumerable<DateTime> holidays = null)
{
	var sign = Math.Sign(days); // if (days == 0) return 0 , if (days > 0) return 1 , if (days < 0) return -1
	var unsignedDays = Math.Abs(days);
	for (var i = 0; i < unsignedDays; i++)
	{
		do
		{
			current = current.AddDays(sign);
		}
		while (current.DayOfWeek == DayOfWeek.Saturday
			|| current.DayOfWeek == DayOfWeek.Sunday
			|| (holidays != null && holidays.Contains(current.Date))
			);
	}
	return current;
}