A simple, light-weight script that sets up a new ActiveSupport::Duration class to handle weekdays.

weekdays.rb

module Weekdays  
  module NumericExtensions
    def weekdays
      Weekday.new(self)
    end
    alias :weekday :weekdays
  end
  
  module TimeExtensions
    def weekday?
      (1..5) === self.wday
    end
  end
  
  class Weekday < ActiveSupport::Duration

    def initialize(days)
      super(days * 24.hours, [[:weekdays, days]])
    end

    def inspect
      parts.
        reduce(::Hash.new(0)) { |h,(l,r)| h[l] += r; h }.
        sort_by {|unit,  _ | [:years, :months, :weekdays, :days, :minutes, :seconds].index(unit)}.
        map     {|unit, val| "#{val} #{val == 1 ? unit.to_s.chop : unit.to_s}"}.
        to_sentence(:locale => :en)
    end

    private

    def sum(sign, time = ::Time.current) #:nodoc:
      parts.inject(time) do |t,(type,number)|
        if t.acts_like?(:time) || t.acts_like?(:date)
          workdays = 0
          while number > 0
            time += 1.day
            workdays += 1
            number -= 1 if time.weekday? ## Only count down if it counts as a weekday
          end
          t.advance(:days => sign * workdays)
        else
          raise ::ArgumentError, "expected a time or date, got #{time.inspect}"
        end
      end
    end
  end
end
  
Numeric.send(:include, Weekdays::NumericExtensions)
Date.send(:include, Weekdays::TimeExtensions)
Time.send(:include, Weekdays::TimeExtensions)