gistfile1.txt

class NullDate
  include Comparable
  def to_s
    ""  
  end 
  def <=> other_date
    Time.new(0000,1,1) <=> other_date
  end 
  def to_date
    ""  
  end 
  def to_datetime

  end 
  def strftime format
    "No data yet."
  end 
  # just in case...
  def blank?
    true
  end 
end

gistfile2.txt

█▓▒░tony@matsuri░▒▓█ ~/src/app(development)$ irb
[1] pry(main)> require './lib/null_date'                                                                                              
=> true
[2] pry(main)> Time.new(2013,3,3) > NullDate.new                                                                                      
=> true
[3] pry(main)> NullDate.ancestors
=> [NullDate, Comparable, Object, PP::ObjectMixin, Kernel, BasicObject]


█▓▒░tony@matsuri░▒▓█ ~/src/app(development)$ ./script/rails c
Loading development environment (Rails 3.2.13)
[1] pry(main)> Time.new(2013,3,3) > NullDate.new
ArgumentError: comparison of Time with NullDate failed
from (pry):1:in `>'
[2] pry(main)> NullDate.ancestors
=> [NullDate,
 Comparable,
 Object,
 Metaclass::ObjectMethods,
 Mocha::ObjectMethods,
 PP::ObjectMixin,
 JSON::Ext::Generator::GeneratorMethods::Object,
 ActiveSupport::Dependencies::Loadable,
 Kernel,
 BasicObject]