Consider the following example:

class Person
include Mongoid::Document
field :updated_at, type: Time
end

p = Person.new
time = DateTime.parse("2012-06-17 18:42:15.123Z")
p.updated_at = time
puts p.updated_at
puts p.updated_at.to_f - time.to_f # should be close to 0

The result on my machine is 2012-06-17 18:42:15 +0200 and -7200.12299990654.

If I look at mongoid/extensions/date_time.rb, Mongoid does the following to convert the DateTime to a Time:

::Time.configured.local(year, month, day, hour, min, sec)

Essentially, it throws away the timezone and any fractions of a second.

Personally I don't care in which timezone the result is (UTC or my local timezone), as long as the actual point in time stays the same. In other words, p.updated_at.to_f should be equal to time.to_f, apart from rounding issues.

This was tested with Mongoid 3.0.0.rc and Ruby 1.9.2p290.