def subtract_business_days(date, delta)
      date = roll_backward(date)
      delta.times do
        begin
          date -= day_interval_for(date)
        end until business_day?(date)
      end
      date
    end
def subtract_business_days(date, delta)
      date = roll_backward(date)
      delta.times do
        begin
          date -= day_interval_for(date)
        end until business_day?(date)
      end
      date
    end
Subtract a number of business days to a date. If a non-business day is
 given, counting will start from the previous business day. So,
   friday - 1 = thursday
   monday - 1 = friday
   sunday - 1 = thursday