mongoid datetime resolution less than a second

gistfile1.rb

require 'rubygems'
require "test/unit"
require 'mongoid'

class DT 
  include Mongoid::Document
  field :birthday, :type => DateTime
end

class TestDT < Test::Unit::TestCase
  def test_mongoid_date_time
    t = Time.utc(2011, 1, 8, 12, 45, 55, 555000) ## 555 milliseconds
    doc = DT.new( :birthday => t )
    assert_equal t.to_f, doc.birthday.to_f
  end
end

<<HERE
$ ruby dt_test.rb 

  1) Failure:

<1294490755.555> expected but was
<1294490755.0>.
HERE

Not that I know about.