different behaviour of datetime.datetime object before vs after save

I'm baffled by the fact that the __str__ method of an instance fails
before the instance is saved but works fine afterwards.

The relevant snippet from my models.py file:
class Journal(models.Model):
date = models.DateTimeField(default=timezone.now)
user = models.CharField(max_length=24)
description = models.CharField(max_length=256)
def __str__(self):
ret = [" #{:0>3} on {:<12} by {}."
.format(self.id,
self.date.strftime('%Y-%m-%d %H:%M'),
self.user)]
for line in self.description.split('\n'):
ret.append(" {}".format(line))
return '\n'.join(ret)

Here's the result of some experimentation using the shell:
(venv)alex@x301:~/Py/debk$ ./manage.py shell
Python 3.4.0 (default, Jun 19 2015, 14:18:46)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from journal.models import Journal
>>> j = Journal(user='Alex', description='Test entry')
>>> j
Traceback (most recent call last):
File "<console>", line 1, in <module>
File
"/home/alex/Py/venv/lib/python3.4/site-packages/django/db/models/base.py",
line 496, in __repr__
u = six.text_type(self)
File "/home/alex/Py/debk/journal/models.py", line 22, in __str__
self.user)]
TypeError: non-empty format string passed to object.__format__
>>> j.save()
>>> j
<Journal: #005 on 2015-09-24 06:11 by Alex.
Test entry>
>>>

I don't understand why saving would change the behavior of an instance.
I'd like my code to be able to display such an instance before a
decision is made whether or not to save it.
I'd be grateful if anyone could explain what's happening and perhaps
suggest a way to use the __str__ method before saving.

Thanks in advance for any help.

Alex K

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