http://rubylearning.com/blog/2010/05/25/rpcfn-business-hours-10

business_hours.rb

require "time"
require "date"

class Date
  def to_time
    Time.local(year, month, day)
  end
end

class Time
  def to_date
    Date.new(year, month, day)
  end
end

class BusinessHours
  def initialize(opening, closing)
    @schedule = { :default => [opening, closing] }
  end
  
  def update(day, opening, closing)
    key = day.kind_of?(Symbol) ? Date.parse(day.to_s).wday : Date.parse(day)
    @schedule[key] = [opening, closing]
  end
  
  def closed(*days)
    days.each { |day| update(day, "0:00", "0:00") }
  end
  
  def calculate_deadline(interval, start_time)
    Deadline.new(@schedule, interval, Time.parse(start_time)).calculate
  end
  
  class Deadline
    def initialize(*args)
      @schedule, @remaining, @current_time = *args
    end
    
    def calculate
      increment_day while after_today?
      current_or_opening_time + @remaining
    end
    
    private
    
    def after_today?
      current_or_opening_time + @remaining > closing_time
    end
    
    def increment_day
      @remaining -= closing_time - current_or_opening_time if @current_time < closing_time
      @current_time = @current_time.to_date.next.to_time
    end
    
    def current_or_opening_time
      [@current_time, opening_time].max
    end
    
    def opening_time
      Time.parse(current_hours.first, @current_time)
    end
    
    def closing_time
      Time.parse(current_hours.last, @current_time)
    end
    
    def current_hours
      @schedule[@current_time.to_date] || @schedule[@current_time.wday] || @schedule[:default]
    end
  end
end

business_hours_test.rb

require "test/unit"
require File.dirname(__FILE__) + '/business_hours'

class BusinessHoursTest < Test::Unit::TestCase
  def setup
    @hours = BusinessHours.new("8:00 AM", "5:00 PM")
  end
  
  def test_within_working_hours
    assert_equal Time.parse("Dec 21, 2009 3:05 PM"), @hours.calculate_deadline(5*60, "Dec 21, 2009 3:00 PM")
  end
  
  def test_start_at_opening_time
    assert_equal Time.parse("Dec 21, 2009 8:05 AM"), @hours.calculate_deadline(5*60, "Dec 21, 2009 7:27 AM")
  end
  
  def test_start_next_day_when_after_closing_time
    assert_equal Time.parse("Dec 21, 2009 8:05 AM"), @hours.calculate_deadline(5*60, "Dec 20, 2009 6:37 PM")
  end
  
  def test_carry_over_remaining_time_onto_next_day
    assert_equal Time.parse("Dec 22, 2009 8:02 AM"), @hours.calculate_deadline(5*60, "Dec 21, 2009 4:57 PM")
  end
  
  def test_skip_full_day
    assert_equal Time.parse("Dec 23, 2009 8:57 AM"), @hours.calculate_deadline(10*60*60, "Dec 21, 2009 4:57 PM")
  end
  
  def test_skip_current_day_before_opening
    assert_equal Time.parse("Dec 22, 2009 9:00 AM"), @hours.calculate_deadline(10*60*60, "Dec 21, 2009 7:57 AM")
  end
  
  def test_update_week_day_hours
    @hours.update :mon, "8:00 AM", "3:00 PM"
    @hours.update :tue, "9:00 AM", "5:00 PM"
    assert_equal Time.parse("Dec 22, 2009 9:02 AM"), @hours.calculate_deadline(5*60, "Dec 21, 2009 2:57 PM")
  end
  
  def test_skip_closed_days
    @hours.closed :sat, :sun
    assert_equal Time.parse("Dec 21, 2009 8:02 AM"), @hours.calculate_deadline(5*60, "Dec 18, 2009 4:57 PM")
  end
  
  def test_change_hours_for_specific_dates
    @hours.update "Dec 24, 2009", "8:00 AM", "3:00 PM"
    @hours.closed :sat, :sun, "Dec 25, 2009"
    assert_equal Time.parse("Dec 28, 2009 8:02 AM"), @hours.calculate_deadline(5*60, "Dec 24, 2009 2:57 PM")
    assert_equal Time.parse("Dec 17, 2009 3:02 PM"), @hours.calculate_deadline(5*60, "Dec 17, 2009 2:57 PM")
  end
  
  def test_start_on_closed_days
    @hours.closed :tue, "Dec 25, 2009"
    assert_equal Time.parse("Dec 23, 2009 8:05 AM"), @hours.calculate_deadline(5*60, "Dec 22, 2009 4:57 PM")
    assert_equal Time.parse("Dec 26, 2009 8:05 AM"), @hours.calculate_deadline(5*60, "Dec 25, 2009 4:57 PM")
  end
end