Getting ruby Date to respect negative offsets when calculating

anchored_date.rb

require 'delegate'
require 'date'

class AnchoredDate < Delegator
  def initialize year, month=1, day=1
    case year
    when Fixnum
      @year = year
      @month = month
      @day = day
      @date = Date.new(@year, @month, @day)
    else
      @date = year.to_date
      @year = @date.year
      @month = @date.month
      @day = @date.day
    end
  end

  def __getobj__
    @date
  end
  
  def + other
    build_clone @date + other
  end
  
  def - other
    build_clone @date - other
  end

  def >> other
    build_clone @date >> other, @day
  end

  def << other
    build_clone @date << other, @day
  end
  
  private
  
  def build_clone new_date, new_day=nil
    new_day ||= @day if @day < 0 && same_day?(new_date)
    self.class.new(new_date.year, new_date.month, new_day || new_date.day)
  end
  
  def same_day? other
    other.day == Date.new(other.year, other.month, @day).day
  end
end