I believe there was a regression when applying this commit. As of now, DateTime fields can still be initialized as before, but can no longer be updated, even when manually converting Time value to UTC.

class TestDate
include Mongoid::Document
field :some_date_time, :type => DateTime
end
> new_date = TestDate.create(:some_date_time => 1.day.ago)
> true
> new_date.update_attribute(:some_date_time, 2.days.ago)
> BSON::InvalidDocument: DateTime is not currently supported; use a UTC Time instance instead.
> new_date.update_attribute(:some_date_time, 2.days.ago.utc)
> BSON::InvalidDocument: DateTime is not currently supported; use a UTC Time instance instead.
> new_date.update_attribute(:some_date_time, Time.now)
> BSON::InvalidDocument: DateTime is not currently supported; use a UTC Time instance instead.