i have this models.py
import datetime
from django.db import models
from tinymce import models as tinymce_models
from filebrowser.fields import FileBrowseField
c
#for sql 'now()' value use
published = models.DateField('Published', auto_now_add=True)
#to allow sql null
published = models.DateField('Published', null=True, blank=True)
You need to add the parameters "null=True, blank=True" to the definition of published, that way it won't be created as a NOT NULL column in the database:
published = models.DateField('Published', null=True, blank=True)