See http://glosoli.blogspot.com/2010/10/beginningoffortnight-method-for-rails.html

beginning_of_fortnight.rb

#
# Extend ActiveSupport to provide a beginning_of_fortnight and an
# end_of_fortnight method for the Time class
#
module ActiveSupport::CoreExtensions::Time::Calculations
  #
  # How many weeks since the beginning_of_week before unix time zero
  #
  def weeks_after_epoch
    ((self - ::Time.at(0).beginning_of_week) / 1.week).to_i
  end

  #
  # The beginning of the current fortnight
  #
  # The choice of which week should begin a fortnight is arbitrary
  # If you want to flip it then change even? to odd? below
  #
  def beginning_of_fortnight
    # If there's been an even number of weeks since epoch use beginning of this week
    # If there's been an odd number of weeks since epoch use the beginning of last week
    (weeks_after_epoch.even? ? self : self - 1.week).beginning_of_week
  end

  #
  # The end of the current fortnight
  #
  def end_of_fortnight
    (beginning_of_fortnight + 13.days).end_of_day
  end
end

Does it work?

[Development]>> puts Time.now; (-5..5).each { |w| t = Time.now + w.weeks; puts "now and #{'%2d'%w} weeks: #{t.beginning_of_fortnight} - #{t.end_of_fortnight}" }; nil
Wed Oct 06 16:29:52 +1000 2010
now and -5 weeks: Mon Aug 30 00:00:00 +1000 2010 - Sun Sep 12 23:59:59 +1000 2010
now and -4 weeks: Mon Aug 30 00:00:00 +1000 2010 - Sun Sep 12 23:59:59 +1000 2010
now and -3 weeks: Mon Sep 13 00:00:00 +1000 2010 - Sun Sep 26 23:59:59 +1000 2010
now and -2 weeks: Mon Sep 13 00:00:00 +1000 2010 - Sun Sep 26 23:59:59 +1000 2010
now and -1 weeks: Mon Sep 27 00:00:00 +1000 2010 - Sun Oct 10 23:59:59 +1000 2010
now and  0 weeks: Mon Sep 27 00:00:00 +1000 2010 - Sun Oct 10 23:59:59 +1000 2010
now and  1 weeks: Mon Oct 11 00:00:00 +1000 2010 - Sun Oct 24 23:59:59 +1000 2010
now and  2 weeks: Mon Oct 11 00:00:00 +1000 2010 - Sun Oct 24 23:59:59 +1000 2010
now and  3 weeks: Mon Oct 25 00:00:00 +1000 2010 - Sun Nov 07 23:59:59 +1000 2010
now and  4 weeks: Mon Oct 25 00:00:00 +1000 2010 - Sun Nov 07 23:59:59 +1000 2010
now and  5 weeks: Mon Nov 08 00:00:00 +1000 2010 - Sun Nov 21 23:59:59 +1000 2010
=> nil