obdev/app/models/access_period.rb

class AccessPeriod < ApplicationRecord
  belongs_to :user
  before_validation :set_default_start_date, on: :create
  validates :end_date, presence: true, if: -> { user&.access_revoked? }

  private

  def set_default_start_date
    self.start_date ||= Date.today if self.new_record? && self.start_date.blank?
  end
end