date_select_to_active_model.rb

class MyObj
  include ActiveModel::Model
  include ActiveRecord::AttributeAssignment

  attr_accessor :my_date

  def initialize(*params)
    assign_attributes(*params)
  end

  # AttributeAssignment needs to know the class
  def type_for_attribute(name)
    case name
    when 'my_date'
      klass = Date
    end
    OpenStruct.new(klass: klass)
  end
end

obj = MyObj.new('my_date(1i)' => '2015', 'my_date(2i)' => '1', 'my_date(3i)' => '31')
obj.my_date # Sat, 31 Jan 2015