It's About Time

Getting to grips with timezones in Django

David Seddon

david@seddonym.me

http://seddonym.me

Do you prefer to have timezones enabled on your sites?

1. Yes

2. No

3. It depends on the site

4. I don't really think about it

Have you ever seen
this warning?

DateTimeField MyModel.my_datetime received a
naive datetime (2016-04-09 16:19:52.557550)
while time zone support is active.

In Django, is timezone support enabled by default?

1. Yes

2. No

3. I have no idea

4. None of the above

Timezones are something we would rather not
think about

This talk

Timezone settings
Naive versus aware datetimes
Datetimes in models, forms
and templates

Part 1

Timezone settings

Two settings you should care about

USE_TZ

Whether or not timezones
are enabled.

TIME_ZONE

The default time zone.


USE_TZ = True


TIME_ZONE = 'Europe/London'

USE_TZ is False by default, but True in projects
created using django-admin.py startproject.

Part 2

Naive vs. aware datetimes

12 April 2016, 7.30pm

A naive datetime

Naive datetimes do not represent moments in time (even if it looks like they do).

Instead, they are calendar events.

Getting a naive datetime

>>> from datetime import datetime

>>> naive_datetime = datetime.now()
>>> naive_datetime
datetime.datetime(2016, 4, 12, 19, 30, 24, 247648)

>>> from django.utils import timezone
>>> timezone.is_naive(naive_datetime)
True

12 April 2016, 7.30pm, London time

Some aware datetimes

Aware datetimes represent moments in time.

12 April 2016, 6.30pm, UTC

12 April 2016, 8.30pm, Paris time

Getting a aware datetime

>>> from django.utils import timezone
>>>
>>> aware_datetime = timezone.now()
>>> aware_datetime
datetime.datetime(2016, 4, 12, 19, 30, 24, 247648, 
tzinfo=<UTC>)

>>> timezone.is_aware(aware_datetime)
True

# settings.py

USE_TZ = True

If USE_TZ is False, timezone.now() will return a naive datetime.

Making datetimes aware

>>> from django.utils import timezone

>>> from datetime import datetime

>>> naive_datetime = datetime.now()

>>> naive_datetime
datetime.datetime(2016, 4, 12, 19, 30, 24, 247648)

>>> aware_datetime = timezone.make_aware(naive_datetime)

>>> aware_datetime
datetime.datetime(2016, 4, 12, 19, 30, 24, 247648, 
tzinfo=<DstTzInfo 'Europe/London' BST+1:00:00 DST>)

make_aware() will use the current timezone by default,
make sure it's the correct one!

Current vs Default timezone

The default time zone is the time zone defined by the TIME_ZONE setting.

The current time zone is the time zone that’s used for rendering. This can be something that your application can set.

# my_app/views.py

import pytz
from django.utils import timezone


def my_view(request):
    timezone.activate(pytz.timezone('Europe/London'))
    # etc.

Dates and times

Dates and times are not datetimes.

They are calendar events, and are always naive in Django.

Don't combine DateFields and TimeFields, to depict datetimes.

Part 3

Datetimes in models, forms and templates

Timezone-aware Django

April 12th 2016, 7.30pm

April 12th 2016, 7.30pm Europe/London

User submits datetime in form

Django assumes the datetime is in the current timezone

April 12th 2016, 6.30pm UTC

Django converts the datetime to UTC before storing in database

April 12th 2016, 6.30pm UTC

Django fetches the datetime from the database, as UTC

April 12th 2016, 7.30pm

Django renders the datetime in
the current timezone