Why are Date types stored as Time strings in MongoDB

Question

Running a Rails app with MongoId 7.1 as ODM. Given this model:

class Memory
  include Mongoid::Document
  field :day_of_event, type: Date
end

Lets say, I create a memory:

m1 = Memory.create!(day_of_event: Date.new(2017,3,2))

If I enter into my console:

m1.day_of_event => Thu, 02 Mar 2017

m1.class => Date

BUT

m1.attributes['day_of_event'] => 2017-03-02 00:00:00 UTC

m1.attributes['day_of_event'].class => Time

m1.read_attribute_before_type_cast('day_of_event').class => Time

The attributes and read_attribute_before_type_cast method return raw_data (bypassing MongoId) which is recognised as Time type in Ruby. (strictly speaking, both should return strings from the db)

I want to store a date, not a point in time. I don't want to see the birthday of my nephew on a different day, because I didn't expect Timezone information in my data. I don't understand, why the UTC information is added here. Why are there Date and Timestamp types, if in the end both are stored as data containing time information?