.. _time-zones:

==========
Time zones
==========

.. versionadded:: 1.4

Overview
========

When support for time zones is enabled, Django stores date and time
information in UTC in the database, uses time zone-aware datetime objects
internally, and translates them to the end user's time zone in templates and
forms.

This is handy if your users live in more than one time zone and you want to
display date and time information according to each user's wall clock. Even if
your website is available in only one time zone, it's still a good practice to
store data in UTC in your database. Here is why.

Many countries have a system of daylight saving time (DST), where clocks are
moved forwards in spring and backwards in autumn. If you're working in local
time, you're likely to encounter errors twice a year, when the transitions
happen. pytz' docs discuss `these issues`_ in greater detail. It probably
doesn't matter for your blog, but it's more annoying if you over-bill or
under-bill your customers by one hour, twice a year, every year. The solution
to this problem is to use UTC in the code and local time only when
interacting with end users.

Time zone support is disabled by default. To enable it, set :setting:`USE_TZ =
True <USE_TZ>` in your settings file. Installing pytz_ is highly recommended,
but not mandatory.

.. note::

    The default :file:`settings.py` file created by :djadmin:`django-admin.py
    startproject <startproject>` includes :setting:`USE_TZ = True <USE_TZ>`
    for convenience.

.. note::

    There is also an independent but related :setting:`USE_L10N` setting that
    controls if Django should activate format localization. See
    :doc:`/topics/i18n/formatting` for more details.

Concepts
========

Naive and aware datetime objects
--------------------------------

Python's :class:`datetime.datetime` objects have a ``tzinfo`` attribute that
can be used to store time zone information, represented as an instance of a
subclass of :class:`datetime.tzinfo`. When this attribute is set and describes
an offset, a datetime object is **aware**; otherwise, it's **naive**.

You can use :func:`~django.utils.timezone.is_aware` and
:func:`~django.utils.timezone.is_naive` to determine if datetimes are aware or
naive.

When time zone support is disabled, Django uses naive datetime objects in local
time. This is simple and sufficient for many use cases. In this mode, to obtain
the current time, you would write::

    import datetime

    now = datetime.datetime.now()

When time zone support is enabled, Django uses time zone aware datetime
objects. If your code creates datetime objects, they should be aware too. In
this mode, the example above becomes::

    import datetime
    from django.utils.timezone import utc

    now = datetime.datetime.utcnow().replace(tzinfo=utc)

.. note::

    :mod:`django.utils.timezone` provides a
    :func:`~django.utils.timezone.now()` function that returns a naive or
    aware datetime object according to the value of :setting:`USE_TZ`.

.. warning::

    Dealing with aware datetime objects isn't always intuitive. For instance,
    the ``tzinfo`` argument of the standard datetime constructor doesn't work
    reliably for time zones with DST. Using UTC is generally safe; if you're
    using other time zones, you should review `pytz' documentation <pytz>`_
    carefully.

.. note::

    Python's :class:`datetime.time` objects also feature a ``tzinfo``
    attribute, and PostgreSQL has a matching ``time with time zone`` type.
    However, as PostgreSQL's docs put it, this type "exhibits properties which
    lead to questionable usefulness".

    Django only supports naive time objects and will raise an exception if you
    attempt to save an aware time object.

.. _naive-datetime-objects:

Interpretation of naive datetime objects
----------------------------------------

When :setting:`USE_TZ` is ``True``, Django still accepts naive datetime
objects, in order to preserve backwards-compatibility. When the database layer
receives one, it attempts to make it aware by interpreting it in the
:ref:`default time zone <default-current-time-zone>` and raises a warning.

Unfortunately, during DST transitions, some datetimes don't exist or are
ambiguous. In such situations, pytz_ raises an exception. Other
:class:`~datetime.tzinfo` implementations, such as the local time zone used as
a fallback when pytz_ isn't installed, may raise an exception or return
inaccurate results. That's why you should always create aware datetime objects
when time zone support is enabled.

In practice, this is rarely an issue. Django gives you aware datetime objects
in the models and forms, and most often, new datetime objects are created from
existing ones through :class:`~datetime.timedelta` arithmetic. The only
datetime that's often created in application code is the current time, and
:func:`timezone.now() <django.utils.timezone.now>` automatically does the
right thing.

.. _default-current-time-zone:

Default time zone and current time zone
---------------------------------------

The **default time zone** is the time zone defined by the :setting:`TIME_ZONE`
setting.

When pytz_ is available, Django loads the definition of the default time zone
from the `tz database`_. This is the most accurate solution. Otherwise, it
relies on the difference between local time and UTC, as reported by the
operating system, to compute conversions. This is less reliable, especially
around DST transitions.

The **current time zone** is the time zone that's used for rendering.

You should set it to the end user's actual time zone with
:func:`~django.utils.timezone.activate`. Otherwise, the default time zone is
used.

.. note::

    As explained in the documentation of :setting:`TIME_ZONE`, Django sets
    environment variables so that its process runs in the default time zone.
    This happens regardless of the value of :setting:`USE_TZ` and of the
    current time zone.

    When :setting:`USE_TZ` is ``True``, this is useful to preserve
    backwards-compatibility with applications that still rely on local time.
    However, :ref:`as explained above <naive-datetime-objects>`, this isn't
    entirely reliable, and you should always work with aware datetimes in UTC
    in your own code. For instance, use
    :meth:`~datetime.datetime.utcfromtimestamp` instead of
    :meth:`~datetime.datetime.fromtimestamp` -- and don't forget to set
    ``tzinfo`` to :data:`~django.utils.timezone.utc`.

Selecting the current time zone
-------------------------------

The current time zone is the equivalent of the current :term:`locale <locale
name>` for translations. However, there's no equivalent of the
``Accept-Language`` HTTP header that Django could use to determine the user's
time zone automatically. Instead, Django provides :ref:`time zone selection
functions <time-zone-selection-functions>`. Use them to build the time zone
selection logic that makes sense for you.

Most websites who care about time zones just ask users in which time zone they
live and store this information in the user's profile. For anonymous users,
they use the time zone of their primary audience or UTC. pytz_ provides
helpers, like a list of time zones per country, that you can use to pre-select
the most likely choices.

Here's an example that stores the current timezone in the session. (It skips
error handling entirely for the sake of simplicity.)

Add the following middleware to :setting:`MIDDLEWARE_CLASSES`::

    from django.utils import timezone

    class TimezoneMiddleware(object):
        def process_request(self, request):
            tz = request.session.get('django_timezone')
            if tz:
                timezone.activate(tz)

Create a view that can set the current timezone::

    import pytz
    from django.shortcuts import redirect, render

    def set_timezone(request):
        if request.method == 'POST':
            request.session[session_key] = pytz.timezone(request.POST['timezone'])
            return redirect('/')
        else:
            return render(request, 'template.html', {'timezones': pytz.common_timezones})

Include in :file:`template.html` a form that will ``POST`` to this view:

.. code-block:: html+django

    {% load tz %}{% load url from future %}
    <form action="{% url 'set_timezone' %}" method="POST">
        {% csrf_token %}
        <label for="timezone">Time zone:</label>
        <select name="timezone">
            {% for tz in timezones %}
            <option value="{{ tz }}"{% if tz == TIME_ZONE %} selected="selected"{% endif %}>{{ tz }}</option>
            {% endfor %}
        </select>
        <input type="submit" value="Set" />
    </form>

Time zone aware input in forms
==============================

When you enable time zone support, Django interprets datetimes entered in
forms in the :ref:`current time zone <default-current-time-zone>` and returns
aware datetime objects in ``cleaned_data``.

If the current time zone raises an exception for datetimes that don't exist or
are ambiguous because they fall in a DST transition (the timezones provided by
pytz_ do this), such datetimes will be reported as invalid values.

.. _time-zones-in-templates:

Time zone aware output in templates
===================================

When you enable time zone support, Django converts aware datetime objects to
the :ref:`current time zone <default-current-time-zone>` when they're rendered
in templates. This behaves very much like :doc:`format localization
</topics/i18n/formatting>`.

.. warning::

    Django doesn't convert naive datetime objects, because they could be
    ambiguous, and because your code should never produce naive datetimes when
    time zone support is enabled. However, you can force conversion with the
    template filters described below.

Conversion to local time isn't always appropriate -- you may be generating
output for computers rather than for humans. The following filters and tags,
provided the ``tz`` template library, allow you to control the time zone
conversions.

Template tags
-------------

.. templatetag:: localtime

localtime
~~~~~~~~~

Enables or disables conversion of aware datetime objects to the current time
zone in the contained block.

This tag has exactly the same effects as the :setting:`USE_TZ` setting as far
as the template engine is concerned. It allows a more fine grained control of
conversion.

To activate or deactivate conversion for a template block, use::

    {% load tz %}

    {% localtime on %}
        {{ value }}
    {% endlocaltime %}

    {% localtime off %}
        {{ value }}
    {% endlocaltime %}

.. note::

    The value of :setting:`USE_TZ` isn't respected inside of a
    ``{% localtime %}`` block.

.. templatetag:: timezone

timezone
~~~~~~~~

Sets or unsets the current time zone in the contained block. When the current
time zone is unset, the default time zone applies.

::

    {% load tz %}

    {% timezone "Europe/Paris" %}
        Paris time: {{ value }}
    {% endtimezone %}

    {% timezone None %}
        Server time: {{ value }}
    {% endtimezone %}

.. note::

    In the second block, ``None`` resolves to the Python object ``None``
    because isn't defined in the template context, not because it's the string
    ``None``.

.. templatetag:: get_current_timezone

get_current_timezone
~~~~~~~~~~~~~~~~~~~~

When the :func:`django.core.context_processors.tz` context processor is
enabled -- by default, it is -- each :class:`~django.template.RequestContext`
contains a ``TIME_ZONE`` variable that provides the name of the current time
zone.

If you don't use a :class:`~django.template.RequestContext`, you can obtain
this value with the ``get_current_timezone`` tag::

    {% get_current_timezone as TIME_ZONE %}

Template filters
----------------

These filters accept both aware and naive datetimes. For conversion purposes,
they assume that naive datetimes are in the default time zone. They always
return aware datetimes.

.. templatefilter:: localtime

localtime
~~~~~~~~~

Forces conversion of a single value to the current time zone.

For example::

    {% load tz %}

    {{ value|localtime }}

.. templatefilter:: utc

utc
~~~

Forces conversion of a single value to UTC.

For example::

    {% load tz %}

    {{ value|utc }}

.. templatefilter:: timezone

timezone
~~~~~~~~

Forces conversion of a single value to an arbitrary timezone.

The argument must be an instance of a :class:`~datetime.tzinfo` subclass or a
time zone name. If it is a time zone name, pytz_ is required.

For example::

    {% load tz %}

    {{ value|timezone:"Europe/Paris" }}

.. _time-zones-migration-guide:

Migration guide
===============

Here's how to migrate a project that was started before Django supported time
zones.

Data
----

PostgreSQL
~~~~~~~~~~

The PostgreSQL backend stores datetimes as ``timestamp with time zone``. In
practice, this means it converts datetimes from the connection's time zone to
UTC on storage, and from UTC to the connection's time zone on retrieval.

As a consequence, if you're using PostgreSQL, you can switch between ``USE_TZ
= False`` and ``USE_TZ = True`` freely. The database connection's time zone
will be set to :setting:`TIME_ZONE` or ``UTC`` respectively, so that Django
obtains correct datetimes in all cases. You don't need to perform any data
conversions.

Other databases
~~~~~~~~~~~~~~~

Other backends store datetimes without time zone information. If you switch
from ``USE_TZ = False`` to ``USE_TZ = True``, you must convert your data from
local time to UTC -- which isn't deterministic if your local time has DST.

Code
----

The first step is to add :setting:`USE_TZ = True <USE_TZ>` to your settings
file and install pytz_ (if possible). At this point, things should mostly
work. If you create naive datetime objects in your code, Django makes them
aware when necessary.

However, these conversions may fail around DST transitions, which means you
aren't getting the full benefits of time zone support yet. Also, you're likely
to run into a few problems because it's impossible to compare a naive datetime
with an aware datetime. Since Django now gives you aware datetimes, you'll get
exceptions wherever you compare a datetime that comes from a model or a form
with a naive datetime that you've created in your code.

So the second step is to refactor your code wherever you instanciate datetime
objects to make them aware. This can be done incrementally.
:mod:`django.utils.timezone` defines some handy helpers for compatibility
code: :func:`~django.utils.timezone.now`,
:func:`~django.utils.timezone.is_aware`,
:func:`~django.utils.timezone.is_naive`,
:func:`~django.utils.timezone.make_aware`, and
:func:`~django.utils.timezone.make_naive`.

Finally, in order to help you locate code that needs upgrading, Django raises
a warning when you attempt to save a naive datetime to the database. During
development, you can turn such warnings into exceptions and get a traceback
by adding to your settings file::

    import warnings
    warnings.filterwarnings(
            'error', r"DateTimeField received a naive datetime",
            RuntimeWarning, r'django\.db\.models\.fields')

.. _pytz: http://pytz.sourceforge.net/
.. _these issues: http://pytz.sourceforge.net/#problems-with-localtime
.. _tz database: http://en.wikipedia.org/wiki/Tz_database
