def add_banking_days(days)
      day = self
      if days > 0
        days.times { day = day.next_banking_day }
      elsif days < 0
        (-days).times { day = day.previous_banking_day }
      end
      day
    end
def add_banking_days(days)
      day = self
      if days > 0
        days.times { day = day.next_banking_day }
      elsif days < 0
        (-days).times { day = day.previous_banking_day }
      end
      day
    end
Adds the given number of banking days, i.e. bank holidays don't count.
 If days is negative, subtracts the given number of banking days.
 If days is zero, returns self.