On a clean install, fresh database (upgraded my dev box), I am seeing issues with naive datetimes:

```
ERROR: test_made_appointment (appointments.tests.test_app.AppointmentAppTestCase)
Mark an appointment as seen.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/daaray/django/rapidsms-appointments/appointments/tests/test_app.py", line 48, in test_made_appointment
    connection=self.connection, timeline=self.timeline, start=yesterday)
  File "/home/daaray/django/rapidsms-appointments/appointments/tests/base.py", line 56, in create_timeline_subscription
    return TimelineSubscription.objects.create(**defaults)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/manager.py", line 137, in create
    return self.get_query_set().create(**kwargs)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/query.py", line 377, in create
    obj.save(force_insert=True, using=self.db)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/base.py", line 463, in save
    self.save_base(using=using, force_insert=force_insert, force_update=force_update)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/base.py", line 551, in save_base
    result = manager._insert([self], fields=fields, return_id=update_pk, using=using, raw=raw)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/manager.py", line 203, in _insert
    return insert_query(self.model, objs, fields, **kwargs)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/query.py", line 1593, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 911, in execute_sql
    for sql, params in self.as_sql():
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 872, in as_sql
    for obj in self.query.objs
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 292, in get_db_prep_save
    prepared=False)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 816, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 801, in get_prep_value
    value = self.to_python(value)
  File "/home/daaray/.virtualenvs/rwanda/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 761, in to_python
    RuntimeWarning)
RuntimeWarning: DateTimeField received a naive datetime (2013-03-11 00:00:00) while time zone support is active.

----------------------------------------------------------------------
Ran 1 test in 0.025s

FAILED (errors=1)
```

Any opposition to converting this particular instance to

```yesterday = now() - - datetime.timedelta(days=self.milestone.offset)```

with the appropriate **now** imports?