NoMethodError on method added to Date class

I added two methods to the Date class and placed it in lib/core_ext, as follows:

class Date
  def self.new_from_hash(hash)
    Date.new flatten_date_array hash
  end

  private
  def self.flatten_date_array(hash)
     %w(1 2 3).map { |e| hash["date(#{e}i)"].to_i }
  end
end

then created a test

require 'test_helper'

class DateTest < ActiveSupport::TestCase
  test 'the truth' do
    assert true
  end

  test 'can create regular Date' do
    date = Date.new
    assert date.acts_like_date?
  end

  test 'date from hash acts like date' do
    hash = ['1i' => 2015, '2i'=> 'February', '3i' => 14]
    date = Date.new_from_hash hash
    assert date.acts_like_date?
  end
end

Now I am getting an error that: Minitest::UnexpectedError: NoMethodError: undefined method 'flatten_date_array' for Date:Class

Did I define my method incorrectly or something? I've even tried moving flatten_date_array method inside new_from_hash and still got the error. I tried creating a test in MiniTest also and got the same error.