DateTime in Rails is missing milliseconds after substracting 1 day, Time is not

gistfile1.rb

$ irb

>> DateTime.now
=> #<DateTime: 2012-10-29T16:00:33+01:00 ((2456230j,54033s,774400000n),+3600s,2299161j)>
>> DateTime.now - 1
=> #<DateTime: 2012-10-28T16:00:37+01:00 ((2456229j,54037s,137431000n),+3600s,2299161j)>

$ rails c
>> DateTime.now.iso8601 9
=> "2012-10-29T16:03:17.237282000+01:00"
>> (DateTime.now - 1.day).iso8601 9
=> "2012-10-28T16:03:21.000000000+01:00"

>> Time.now.iso8601 9
=> "2012-10-29T16:03:59.672321000+01:00"
>> (Time.now - 1.day).iso8601 9
=> "2012-10-28T16:04:08.504329000+01:00"