django modelのDateTimeField DBカラムデフォルト値でWARNING

(fields.W161) Fixed default value provided.
    HINT: It seems you set a fixed date / time / datetime value as default for this field. This may not be what you want. If you want to have the current date as default, use `django.utils.timezone.now`

# -*- coding: utf-8 -*-
from django.db import models


class Test(models.Model):

    create_date = \
        models.DateTimeField(default=datetime.utcnow())

    update_date = \
        models.DateTimeField(default=datetime.utcnow())

models.DateTimeField(default=datetime.utcnow())

        migrations.AlterField(
            model_name='xxxTableName',
            name='create_date',
            field=models.DateTimeField(default=datetime.datetime(2015, 12, 26, 19, 46, 4, 718301)),
        ),

# -*- coding: utf-8 -*-
from django.db import models
from django.utils.timezone import now


class Test(models.Model):

    create_date = models.DateTimeField(default=now)

    update_date = models.DateTimeField(default=now)

        migrations.AlterField(
            model_name='xxxTableName',
            name='create_date',
            field=models.DateTimeField(default=django.utils.timezone.now),
        )

    Column     |           Type           |                          Modifiers                           
---------------+--------------------------+--------------------------------------------------------------
 id            | integer                  | not null default nextval('xxx_id_seq'::regclass)
 create_date   | timestamp with time zone | not null
 update_date   | timestamp with time zone | not null