# Calculate the number of weekdays between dates
      workdays = 0
      date_has_weekend = false
      date = self.start_at    
      while date.to_date < finish_at.to_date
        #If we have a weekend day, increase finish date
        if date.to_date.wday == 6 or date.to_date.wday == 0
          date_has_weekend = true
          finish_at = finish_at.to_time.advance(:days => 1)          
        end
        workdays = workdays + 1 unless date.to_date.wday == 0 or date.to_date.wday == 6
        date = date.to_time.advance(:days => 1)
      end

Calculate workdays in a date range

date

Ruby