Functional test failing on post :create! TypeError: expected numeric or date

error

  1) UsersController create action should destroy session if is present
     Failure/Error: post :create
     TypeError:
       expected numeric or date
     # ./app/models/user.rb:48:in `user_bigger_than_18'
     # ./app/controllers/users_controller.rb:11:in `create'
     # ./spec/controllers/users_controller_spec.rb:67:in `block (2 levels) in <top (required)>'

users.rb

### USER MODEL


class User < ActiveRecord::Base
  # new columns need to be added here to be writable through mass assignment
  attr_accessible :name, :surname, :email, :password, :password_confirmation, :gender,:cooking_passion_description, :cookous_inspiration, :current_activity, :date_of_birth,:favourite_dishes, :people_i_enjoy, :personal_interests, :personal_statement,:visible_current_activity, :visible_date_of_birth, :visible_lives_in, :confirmation_code, :confirmed
  attr_accessor :password
  
  before_save :prepare_password
  before_create :prepare_confirmation
  
  validates_presence_of :name, :surname, :gender, :date_of_birth
  validates_uniqueness_of :email, :allow_blank => true
  validates_format_of :name, :surname, :with => /^[a-zA-Z]+$/, :allow_blank => true, :message => "should only contain letters"
  validates_format_of :email, :with => /^[-a-z0-9_+\.]+\@([-a-z0-9]+\.)+[a-z0-9]{2,4}$/i
  validates_presence_of :password, :on => :create
  validates_confirmation_of :password
  validates_length_of :password, :minimum => 4, :allow_blank => true
  validates_inclusion_of :gender, :in => %w( male female unknown ), :on => :create, :message => "Please select valid gender"

  validate :user_bigger_than_18
    
  # login can be either username or email address
  def self.authenticate(login, pass)
    user = find_by_email(login)
    return user if user && user.password_hash == user.encrypt_password(pass) && user.confirmed
  end
  
  def self.confirm_user_with_confirmation_code(confirmation)
    u = User.find_by_confirmation_code(confirmation)
    return false unless u   
    u.update_attributes :confirmed => true, :confirmation_code => nil
    
    u.save    
    return u
  end
  
  
  def full_name
    return false unless self.name && self.surname
    return self.name + " " + self.surname
  end
  
  def encrypt_password(pass)
    BCrypt::Engine.hash_secret(pass, password_salt)
  end

  private
  
  def user_bigger_than_18
    days_difference = (Date.today - self.date_of_birth).to_i
    if (days_difference/365 < 18)
      errors.add(:date_of_birth, "You must be over 18 to be able to use Cookous") 
    end
  end
  
  def prepare_confirmation
    unless confirmed == true
      self.confirmed = false
      self.confirmation_code = ActiveSupport::SecureRandom.hex(16)
    end
  end
  
  def prepare_password
    unless password.blank?
      self.password_salt = BCrypt::Engine.generate_salt
      self.password_hash = encrypt_password(password)
    end
  end
end

users_controller_spec.rb

  it "create action should destroy session if is present" do
    session['user_id'] = 128903
    post :create
    
    session['user_id'].should be_nil
  end