Rails : createでsaveされない

非常に基本的なことだと思いますが、何故か新規modelがcreateされません。
原因として考えられるのは、migration fileは日時をdatetimeとしている一方で、
入力の便宜上formではday partとtime partに分けています。ただし、その為の処理はしっかり書けているとと毛のですが、原因がわかりません。

ruby
1##tender model
2
3id         :bigint(8)        not null, primary key
4seller_id  :bigint(8)        not null
5starts_at  :datetime         not null
6ends_at    :datetime         not null
7status     :string           default("draft"), not null
8created_at :datetime         not null
9updated_at :datetime         not null
10
11
12class Tender < ApplicationRecord
13  belongs_to :seller
14
15  STATUS_VALUES = %w(draft ready ongoing finished)
16
17  validates :starts_at, presence: true
18  validates :ends_at, presence: true
19  validates :status, inclusion: {in: STATUS_VALUES}
20
21  scope :open, -> { where(status: "ready")}
22
23  before_save do
24    if starts_at_date_part && starts_at_time_part
25      self.starts_at = "#{starts_at_date_part} #{starts_at_time_part}"
26    end
27
28    if ends_at_date_part && ends_at_time_part
29      self.ends_at = "#{ends_at_date_part} #{ends_at_time_part}"
30    end
31  end
32
33  attr_writer :starts_at_date_part, :starts_at_time_part,
34  :ends_at_date_part, :ends_at_time_part
35
36  def starts_at_date_part
37    @starts_at_date_part ||= starts_at&.strftime('%Y-%m-%d')
38  end
39
40  def starts_at_time_part
41    @starts_at_time_part ||= starts_at&.strftime('%H:%M')
42  end
43
44  def ends_at_date_part
45    @ends_at_date_part ||= ends_at&.strftime('%Y-%m-%d')
46  end
47
48  def ends_at_time_part
49    @ends_at_time_part ||= ends_at&.strftime('%H:%M')
50  end
51
52end
53
54
55##Controller
56  def new
57    @tender = Tender.new
58    time0 = Time.current.beginning_of_day
59    @tender.starts_at = time0.advance(days: 1, hours: 10)
60    @tender.ends_at = time0.advance(days: 3, hours: 15)
61  end
62
63  def create
64    @tender = Tender.new(tender_params)
65    @tender.seller = current_seller
66    if @tender.save
67      binding.pry
68      flash.notice = "入札会を設定しました。"
69      redirect_to :sellers_tenders
70    else
71      render "new"
72    end
73  end
74
75
76##form
77
78<%= form_for @tender, url: :sellers_tenders do |f| %>
79
80        <div class="form-group">
81          <%= f.label :starts_at_date_part, "入札「開始」日"%>&nbsp;&nbsp;&nbsp;<span class="badge badge-pill badge-danger">必須</span>
82          <div class='input-group'>
83            <div class="input-group-prepend">
84              <span class="input-group-text"><i class='far fa-calendar-alt'></i></span>
85            </div>
86            <%= f.text_field :starts_at_date_part, class: 'form-control date-picker', required: true %>
87          </div>
88        </div>
89        <div class="form-group" >
90          <%= f.label :starts_at_time_part, "入札「開始」時間"%>&nbsp;&nbsp;&nbsp;<span class="badge badge-pill badge-danger">必須</span>
91          <div class='input-group'>
92            <div class="input-group-prepend">
93              <span class="input-group-text"><i class='far fa-clock'></i></span>
94            </div>
95            <%= f.text_field :starts_at_time_part, class: 'form-control time-picker', required: true %>
96          </div>
97        </div>
98
99        <div class="form-group" >
100          <%= f.label :ends_at_date_part, "入札「終了」日"%>&nbsp;&nbsp;&nbsp;<span class="badge badge-pill badge-danger">必須</span>
101          <div class='input-group'>
102            <div class="input-group-prepend">
103              <span class="input-group-text"><i class='far fa-calendar-alt'></i></span>
104            </div>
105            <%= f.text_field :ends_at_date_part, class: 'form-control date-picker', required: true %>
106          </div>
107        </div>
108        <div class="form-group" >
109          <%= f.label :ends_at_time_part, "入札「終了」時間" %>&nbsp;&nbsp;&nbsp;<span class="badge badge-pill badge-danger">必須</span>
110          <div class='input-group'>
111            <div class="input-group-prepend">
112              <span class="input-group-text"><i class='far fa-clock'></i></span>
113            </div>
114            <%= f.text_field :ends_at_time_part, class: 'form-control time-picker', required: true %>
115          </div>
116        </div>
117
118        <div class="form-group mt-5" >
119          <%= f.submit "入札会を設定する", class: 'btn btn-success' %>
120        </div>
121
122        <% end %>
123