Example of using an object to build dates and times using individual fields

_appointment_time.html.erb

<%= f.label :appointment_time %><br>
<%= f.fields_for :appointment_time, @person.appointment_time do |t| %>
  <%= t.text_field :day, size: 2, placeholder: 'DD' %> /
  <%= t.text_field :month, size: 2, placeholder: 'MM' %> /
  <%= t.text_field :year, size: 4, placeholder: 'YYYY' %> -
  <%= t.text_field :hour, size: 2, placeholder: 'HH' %> :
  <%= t.text_field :min, size: 2, placeholder: 'MM' %>
<% end %>

_date_of_birth.html.erb

<%= f.label :date_of_birth %><br>
<%= f.fields_for :date_of_birth, @person.date_of_birth do |t| %>
  <%= t.text_field :day, size: 2, placeholder: 'DD' %> /
  <%= t.text_field :month, size: 2, placeholder: 'MM' %> /
  <%= t.text_field :year, size: 4, placeholder: 'YYYY' %> -
<% end %>

date_builder.rb

class DateBuilder
  attr_reader :day, :month, :year

  def initialize(components)
    today = Date.current

    @day   = components.fetch('day', today.day)
    @month = components.fetch('month', today.month)
    @year  = components.fetch('year', now.year)
  end

  def valid?
    return false unless day.to_s =~ /\A\d{1,2}\z/
    return false unless month.to_s =~ /\A\d{1,2}\z/
    return false unless year.to_s =~ /\A\d{4}\z/

    begin
      build_value
    rescue ArgumentError => e
      false
    end
  end

  def value
    if datetime = valid?
      datetime
    end
  end

  def blank?
    [year, month, day].all?(&:blank?)
  end

  def present?
    !blank?
  end

  private

  def build_value
    Date.new(*arguments)
  end

  def arguments
    [year, month, day].map(&:to_i)
  end
end

date_time_builder.rb

class DateTimeBuilder
  attr_reader :day, :month, :year
  attr_reader :hour, :min, :sec

  def initialize(components)
    now = Time.current

    @day   = components.fetch('day', now.day)
    @month = components.fetch('month', now.month)
    @year  = components.fetch('year', now.year)
    @hour  = components.fetch('hour', now.hour)
    @min   = components.fetch('min', now.min)
    @sec   = components.fetch('sec', 0)
  end

  def valid?
    return false unless day.to_s =~ /\A\d{1,2}\z/
    return false unless month.to_s =~ /\A\d{1,2}\z/
    return false unless year.to_s =~ /\A\d{4}\z/

    return false unless hour.to_s =~ /\A\d{1,2}\z/
    return false unless min.to_s =~ /\A\d{1,2}\z/
    return false unless sec.to_s =~ /\A\d{1,2}\z/

    begin
      build_value
    rescue ArgumentError => e
      false
    end
  end

  def value
    if datetime = valid?
      datetime
    end
  end

  def blank?
    [year, month, day, hour, min, sec].all?(&:blank?)
  end

  def present?
    !blank?
  end

  private

  def build_value
    Time.zone.local(*arguments)
  end

  def arguments
    [year, month, day, hour, min, sec].map(&:to_i)
  end
end

people_controller.rb

class PeopleController < ApplicationController
  private

    def person_params
      params.require(:person).permit(
        :name,
        :date_of_birth => %i[day month year],
        :appointment_time => %i[day month year hour min sec]
      )
    end
end

person.rb

class Person < ActiveRecord::Base

  validates :name, presence: true, length: { maximum: 255 }
  validates :date_of_birth, presence: true
  validates :appointment_time, presence: true

  validate do
    if @date_of_birth_builder && @date_of_birth_builder.present?
      unless @date_of_birth_builder.valid?
        errors.add :date_of_birth, :invalid
      end
    end
  end

  validate do
    if @appointment_time_builder && @appointment_time_builder.present?
      unless @appointment_time_builder.valid?
        errors.add :appointment_time, :invalid
      end
    end
  end

  def date_of_birth=(new_date)
    @date_of_birth_builder = DateBuilder.new(new_date)

    if @date_of_birth_builder.valid?
      super(@date_of_birth_builder.value)
    end
  end

  def date_of_birth
    if @date_of_birth_builder
      @date_of_birth_builder
    else
      super
    end
  end

  def appointment_time=(new_time)
    @appointment_time_builder = DateTimeBuilder.new(new_time)

    if @appointment_time_builder.valid?
      super(@appointment_time_builder.value)
    end
  end

  def appointment_time
    if @appointment_time_builder
      @appointment_time_builder
    else
      super
    end
  end

end