field :date, type: Time
Finally, have a look at the mongoid-metastamp gem. It will give you much better support for querying across multiple timezones, while still seamlessly working like a native Time field.
It looks like you need to specify the field type of your date attribute. I would use a Time field if you want mongoid to handle the zones properly.
This sounds counterintuitive, but this is the current way to make mongoid use UTC as the default timezone.
This works with DateTime too in the current versions of Mongoid (I'm on 2.4.7). See github.com/mongoid/mongoid/issues/1135
You will also probably want to set the following in config/mongoid.yml