date_selectタグを使ってユーザーの誕生日を保存したい

前提・実現したいこと

'devise'を使ってユーザーの新規登録機能を実装しています。
誕生日の入力をdate_selectタグを使って記述し、createアクションを実行したところ以下のようなエラーが表示されます。
エラーを解決し、誕生日の情報をuserテーブルに保存できるようにしたい。
paramsの中身を日付型にする記述をコントローラーないでしていますがうまくいきません。

発生している問題・エラーメッセージ

リンク内容
#####NoMethodError in SignupController#create
######undefined method `[]' for nil:NilClass

ruby
1#signup_controller.rb
2
3 def birthday_join
4      date = params[:user][:birthday]  
5      Date.new date["birthday(1i)"].to_i,date["birthday(2i)"].to_i,date["birthday(3i)"].to_i
6    end
7end

該当のソースコード

ruby
1#signup_controller
2
3class SignupController < ApplicationController
4  def index
5  end
6    
7  def step1
8    @user = User.new
9  end
10  
11  def step2
12    session[:nickname] = user_params[:nickname]
13    session[:email] = user_params[:email]
14    session[:password] = user_params[:password]
15    session[:password_confirmation] = user_params[:password_confirmation]
16    session[:last_name] = user_params[:last_name]
17    session[:first_name] = user_params[:first_name]
18    session[:last_name_kana] = user_params[:last_name_kana]
19    session[:first_name_kana] = user_params[:first_name_kana]
20    session[:birthday] = user_params[:birthday]
21    @user = User.new
22  end
23
24  def create
25    params[:user][:birthday] = birthday_join
26    @user = User.new(
27      nickname: session[:nickname],
28      email: session[:email],
29      password: session[:password],
30      password_confirmation: session[:password_confirmation],
31      last_name: session[:last_name],
32      first_name: session[:first_name],
33      last_name_kana: session[:last_name_kana],
34      first_name_kana: session[:first_name_kana],
35      birthday: session[:birthday],
36      phone_number: session[:phone_number],
37      address_last_name: session[:address_last_name],
38      address_first_name: session[:address_first_name],
39      address_last_name_kana: session[:address_last_name_kana],
40      address_first_name_kana: session[:address_first_name_kana],
41      address_number: session[:address_number],
42      address_prefecture: session[:address_prefecture],
43      address_name: session[:address_name],
44      address_block: session[:address_block],
45      address_building: session[:address_building],
46      address_phone_number: session[:address_phone_number]
47    )
48    if @user.save
49      session[:id] = @user.id
50      redirect_to done_signup_index_path
51    else
52      render signup_index_path
53    end
54  end
55
56  def done
57    sign_in User.find(session[:id]) unless user_sigend_in?
58  end
59
60  private
61    def user_params
62      params.require(:user).permit(
63        :nickname,
64        :email,
65        :password,
66        :password_confirmation,
67        :last_name,
68        :first_name,
69        :last_name_kana,
70        :first_name_kana,
71        :birthday,
72        :phone_number,
73        :address_last_name,
74        :address_first_name,
75        :address_last_name_kana,
76        :address_first_name_kana,
77        :address_number,
78        :address_prefecture,
79        :address_name,
80        :address_block,
81        :address_building,
82        :address_phone_number
83      )
84    end
85    
86    def birthday_join #paramsの中身を日付型にする記述
87      date = params[:user][:birthday]  
88      Date.new date["birthday(1i)"].to_i,date["birthday(2i)"].to_i,date["birthday(3i)"].to_i
89    end
90end

ruby
1#db/migrate/devise_create_users.rb
2
3class DeviseCreateUsers < ActiveRecord::Migration[6.0]
4  def change
5    create_table :users do |t|
6      ## Database authenticatable
7      t.string :nickname,           null: false
8      t.string :email,              null: false, unique: true
9      t.string :last_name,          null: false
10      t.string :first_name,         null: false
11      t.string :last_name_kana,     null: false
12      t.string :first_name_kana,    null: false
13      t.date :birthday,             null: false
14      t.integer :phone_number,      null: false, unique: true
15      t.string :address_last_name,  null: false
16      t.string :address_first_name, null: false
17      t.string :address_last_name_kana,  null: false
18      t.string :address_first_name_kana, null: false
19      t.string :address_number,      null: false
20      t.integer :address_prefecture, null: false, default: 0
21      t.string :address_name,        null: false
22      t.string :address_block,       null: false
23      t.string :address_building
24      t.integer :address_phone_number
25      t.string :encrypted_password,  null: false, default: ""
26
27      ## Recoverable
28      t.string   :reset_password_token
29      t.datetime :reset_password_sent_at
30
31      ## Rememberable
32      t.datetime :remember_created_at
33      t.timestamps null: false
34    end
35
36    add_index :users, :email,                unique: true
37    add_index :users, :reset_password_token, unique: true
38  end
39end

haml
1#step2.html.haml
2
3%body
4  %noscript
5    = image_tag 'logo.svg', alt: 'ee'
6  .single-container
7    %header.single-header
8      %h1.header
9        = link_to "#", class:'link' do
10          = image_tag 'logo.svg', alt: 'mercari', class: 'mercari'
11        %nav.progress-bar.singup-bar
12    %main.single-main
13      %section.l-single-container
14        %h2.l-single-head.registration 住所入力
15        = form_for @user, url: '/signup' , method: :post, html: {class: 'l-single-inner'} do |f|
16          %input(type="hidden")
17          .l-single-content
18            .form-group.nickname
19              = f.label :phone_number, "電話番号"
20              %span.form-require 必須
21              = f.number_field :phone_number, placeholder:"例）080-1234-5678", class:"input-default"
22            .form-group.email
23              = f.label :address_number, "郵便番号"
24              %span.form-require 必須
25              = f.number_field :address_number, placeholder:"123-4567", class:"input-default"
26              %input(type="hidden")
27            .form-group.form-password-revelation
28              = f.label :address_prefecture, "都道府県"
29              %span.form-require 必須
30              = f.text_field :address_prefecture, placeholder:"東京都", class:"input-default"
31              %p.form-info-text
32              = f.label :address_name, "住所"
33              %span.form-require 必須
34              = f.text_field :address_name, placeholder:"港区六本木", class:"input-default"
35              %p.form-info-text
36              = f.label :address_block, "番地"
37              %span.form-require 必須
38              = f.text_field :address_block, placeholder:"1-123-4", class:"input-default"
39              .form-password-revelation-taggle
40                .form-password-revelation-revealed-password-container
41                  %span.form-password-revelation-revealed-password
42            .form-group.confirmation
43              %h3.text-left 本人確認
44              %p.l-single-text
45                安心・安全にご利用いただくために、お客様の本人情報の登録にご協力ください。他のお客様に公開されることはありません。
46            .form-group.name-kanji
47              %div
48                = f.label "送付先お名前",class:"kanji"
49                %span.form-require 必須
50              = f.text_field :address_last_name, placeholder:"例） 山田", class:"input-default half1"
51              = f.text_field :address_first_name, placeholder:"例） 彩", class:"input-default half2"
52            .form-group.name-kana
53              %div
54                = f.label "送付先お名前カナ(全角)",class:"kana"
55                %span.form-require 必須
56              = f.text_field :address_last_name_kana, placeholder:"例） ヤマダ", class:"input-default half3"
57              = f.text_field :address_first_name_kana, placeholder:"例） アヤ",class:"input-default half4"
58              %input(type="hidden")
59              .clearfit
60              %p.form-info-text
61                ※ 本人情報は正しく入力してください。会員登録後、修正するにはお時間をいただく場合があります。
62          .l-single-content2
63            %input(type="hidden")
64            = f.submit "登録する",class:"submit"