module BusinessDay
  def next_business_day
    self.skip_weekends self, 1
  end
  def previous_business_day
    self.skip_weekends self, -1
  end
  protected
  def skip_weekends(date, inc)
    date += inc
    while (date.wday % 7 == 0) or (date.wday % 7 == 6) do
      date += inc
    end
    date
  end
end

Date.send :include, BusinessDay