# settings.py
TIME_ZONE = 'America/Chicago'
AUTH_USER_MODEL = 'app.User'
MIDDLEWARE = [
    ...,
    'django.contrib.sessions.middleware.SessionMiddleware',
    ...,
    'sundial.middleware.TimezoneMiddleware',
    ...,
]

# app/models.py
from django.conf import settings
from django.contrib.auth.models import AbstractUser
from django.contrib.auth.signals import user_logged_in
from django.dispatch.dispatcher import receiver

from sundial.fields import TimezoneField
from sundial.utils import set_session_timezone
from sundial.zones import COMMON_GROUPED_CHOICES

class User(AbstractUser):
    timezone = TimezoneField(
        default=settings.TIME_ZONE, choices=COMMON_GROUPED_CHOICES
    )

@receiver(user_logged_in)
def assign_user_timezone(request, user, **kwargs):
    set_session_timezone(request.session, user.timezone)