Как импортировать модуль datetime в Django?

Я' Мы изучили некоторые основы в Django, но я застрял в этом уроке:

http://docs.djangoproject.com/en/dev/intro/tutorial02/#customize-the-admin-change-list

Обнаружена ошибка, когда was_published_today добавлено:

global name 'datetime' is not defined

Некоторые результаты поиска предлагают мне импортировать модуль datetime, но я понятия не имею, как это сделать .

из django.db импортирует datetime в polls / admin.py или polls / models.py кажется бесполезным

Это settings.py :

# Django settings for kaka project.

DEBUG = True
TEMPLATE_DEBUG = DEBUG

ADMINS = (
    # ('Your Name', 'your_email@domain.com'),
)

MANAGERS = ADMINS

DATABASE_ENGINE = 'mysql'           # 'postgresql_psycopg2', 'postgresql', 'mysql', 'sqlite3' or 'oracle'.
DATABASE_NAME = 'kaka'             # Or path to database file if using sqlite3.
DATABASE_USER = 'root'             # Not used with sqlite3.
DATABASE_PASSWORD = 'z'         # Not used with sqlite3.
DATABASE_HOST = ''             # Set to empty string for localhost. Not used with sqlite3.
DATABASE_PORT = ''             # Set to empty string for default. Not used with sqlite3.

# Local time zone for this installation. Choices can be found here:
# http://en.wikipedia.org/wiki/List_of_tz_zones_by_name
# although not all choices may be available on all operating systems.
# If running in a Windows environment this must be set to the same as your
# system time zone.
TIME_ZONE = 'America/Chicago'

# Language code for this installation. All choices can be found here:
# http://www.i18nguy.com/unicode/language-identifiers.html
LANGUAGE_CODE = 'en-us'

SITE_ID = 1

# If you set this to False, Django will make some optimizations so as not
# to load the internationalization machinery.
USE_I18N = True

# Absolute path to the directory that holds media.
# Example: "/home/media/media.lawrence.com/"
MEDIA_ROOT = ''

# URL that handles the media served from MEDIA_ROOT. Make sure to use a
# trailing slash if there is a path component (optional in other cases).
# Examples: "http://media.lawrence.com", "http://example.com/media/"
MEDIA_URL = ''

# URL prefix for admin media -- CSS, JavaScript and images. Make sure to use a
# trailing slash.
# Examples: "http://foo.com/media/", "/media/".
ADMIN_MEDIA_PREFIX = '/media/'

# Make this unique, and don't share it with anybody.
SECRET_KEY = '5vqk*i^#8%=nkhy46z9uf7vx@5g*0e6#uons*+gb^iakgg8y$('

# List of callables that know how to import templates from various sources.
TEMPLATE_LOADERS = (
    'django.template.loaders.filesystem.load_template_source',
    'django.template.loaders.app_directories.load_template_source',
#     'django.template.loaders.eggs.load_template_source',
)

MIDDLEWARE_CLASSES = (
    'django.middleware.common.CommonMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
)

ROOT_URLCONF = 'kaka.urls'

TEMPLATE_DIRS = (
    "/var/mycode/cina", # Change this to your own directory.
    # Put strings here, like "/home/html/django_templates" or "C:/www/django/templates".
    # Always use forward slashes, even on Windows.
    # Don't forget to use absolute paths, not relative paths.
)

INSTALLED_APPS = (
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.sites',
    'kaka.polls',
    'django.contrib.admin'
)

Это polls / models.py :

from django.db import models

class Poll(models.Model):
    # ...
    def __unicode__(self):
        return self.question
    question = models.CharField(max_length=200)
    def was_published_today(self):
        return self.pub_date.date() == datetime.date.today()
    was_published_today.short_description = 'Published today?'
    pub_date = models.DateTimeField('date published')

class Choice(models.Model):
    # ...
    def __unicode__(self):
        return self.choice
    poll = models.ForeignKey(Poll)
    choice = models.CharField(max_length=200)
    votes = models.IntegerField()

Это polls / admin.py :

from kaka.polls.models import Poll
from kaka.polls.models import Choice
from django.contrib import admin

class ChoiceInline(admin.TabularInline):
    model = Choice
    extra = 3
class PollAdmin(admin.ModelAdmin):
    fieldsets = [
        (None,               {'fields': ['question']}),
        ('Date information', {'fields': ['pub_date'], 'classes': ['collapse']}),
    ]
    list_display = ('question', 'pub_date', 'was_published_today')
    inlines = [ChoiceInline]
    list_filter = ['pub_date']
    search_fields = ['question']
    date_hierarchy = 'pub_date'
admin.site.register(Poll, PollAdmin)

admin.site.register(Choice)