==========

Time zones

==========

.. _time-zones-overview:

Overview

========

When support for time zones is enabled, Django stores datetime 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 datetime information according to each user's wall clock.

Even if your website is available in only one time zone, it's still good

practice to store data in UTC in your database. The main reason is daylight

saving time (DST). Many countries have a system of DST, where clocks are moved

forward in spring and backward in autumn. If you're working in local time,

you're likely to encounter errors twice a year, when the transitions happen.

This probably doesn't matter for your blog, but it's a problem 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 use 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.

.. note::

    In Django 5.0, time zone support will be enabled by default.

Time zone support uses :mod:`zoneinfo`, which is part of the Python standard

library from Python 3.9.  The ``backports.zoneinfo`` package is automatically

installed alongside Django if you are using Python 3.8.

.. versionchanged:: 4.0

    :mod:`zoneinfo` was made the default timezone implementation. You may

    continue to use `pytz`_ during the 4.x release cycle via the

    :setting:`USE_DEPRECATED_PYTZ` setting.

.. note::

    The default :file:`settings.py` file created by :djadmin:`django-admin

    startproject <startproject>` includes :setting:`USE_TZ = True <USE_TZ>`

    for convenience.

If you're wrestling with a particular problem, start with the :ref:`time zone

FAQ <time-zones-faq>`.

Concepts

========

.. _naive_vs_aware_datetimes:

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 whether datetimes are

aware or naive.

When time zone support is disabled, Django uses naive datetime objects in local

time. This is 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 (:setting:`USE_TZ=True <USE_TZ>`), 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::

    from django.utils import timezone

    now = timezone.now()

.. 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 the :mod:`zoneinfo`

    documentation 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, as a timezone for a time with no

    associated date does not make sense.

.. _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. That's why you should always create aware datetime objects when time

zone support is enabled. (See the :mod:`Using ZoneInfo section of the zoneinfo

docs <zoneinfo>` for examples using the ``fold`` attribute to specify the

offset that should apply to a datetime during a DST transition.)

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.

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

You should set the current time zone 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.fromtimestamp`

    and set the ``tz`` parameter to :attr:`~datetime.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 that care about time zones 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.

:func:`zoneinfo.available_timezones` provides a set of available timezones that

you can use to build a map from likely locations to time zones.

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`::

    import zoneinfo

    from django.utils import timezone

    class TimezoneMiddleware:

        def __init__(self, get_response):

            self.get_response = get_response

        def __call__(self, request):

            tzname = request.session.get('django_timezone')

            if tzname:

                timezone.activate(zoneinfo.ZoneInfo(tzname))

            else:

                timezone.deactivate()

            return self.get_response(request)

Create a view that can set the current timezone::

    from django.shortcuts import redirect, render

    # Prepare a map of common locations to timezone choices you wish to offer.

    common_timezones = {

        'London': 'Europe/London',

        'Paris': 'Europe/Paris',

        'New York': 'America/New_York',

    }

    def set_timezone(request):

        if request.method == 'POST':

            request.session['django_timezone'] = request.POST['timezone']

            return redirect('/')

        else:

            return render(request, 'template.html', {'timezones': common_timezones})

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

.. code-block:: html+django

    {% load tz %}

    {% get_current_timezone as TIME_ZONE %}

    <form action="{% url 'set_timezone' %}" method="POST">

        {% csrf_token %}

        <label for="timezone">Time zone:</label>

        <select name="timezone">

            {% for city, tz in timezones %}

            <option value="{{ tz }}"{% if tz == TIME_ZONE %} selected{% endif %}>{{ city }}</option>

            {% endfor %}

        </select>

        <input type="submit" value="Set">

    </form>

.. _time-zones-in-forms:

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``.

Converted datetimes that don't exist or are ambiguous because they fall in a

DST transition 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 by the ``tz`` template tag library, allow you to control the time zone

conversions.

.. highlight:: html+django

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 %}

.. templatetag:: get_current_timezone

``get_current_timezone``

~~~~~~~~~~~~~~~~~~~~~~~~

You can get the name of the current time zone using the

``get_current_timezone`` tag::

    {% get_current_timezone as TIME_ZONE %}

Alternatively, you can activate the

:func:`~django.template.context_processors.tz` context processor and

use the ``TIME_ZONE`` context variable.

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.

For example::

    {% load tz %}

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

.. highlight:: python

.. _time-zones-migration-guide:

Migration guide

===============

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

zones.

Database

--------

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. 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 instantiate 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::

    RuntimeWarning: DateTimeField ModelName.field_name received a naive

    datetime (2012-01-01 00:00:00) while time zone support is active.

During development, you can turn such warnings into exceptions and get a

traceback by adding the following to your settings file::

    import warnings

    warnings.filterwarnings(

        'error', r"DateTimeField .* received a naive datetime",

        RuntimeWarning, r'django\.db\.models\.fields',

    )

Fixtures

--------

When serializing an aware datetime, the UTC offset is included, like this::

    "2011-09-01T13:20:30+03:00"

While for a naive datetime, it isn't::

    "2011-09-01T13:20:30"

For models with :class:`~django.db.models.DateTimeField`\ s, this difference

makes it impossible to write a fixture that works both with and without time

zone support.

Fixtures generated with ``USE_TZ = False``, or before Django 1.4, use the

"naive" format. If your project contains such fixtures, after you enable time

zone support, you'll see :exc:`RuntimeWarning`\ s when you load them. To get

rid of the warnings, you must convert your fixtures to the "aware" format.

You can regenerate fixtures with :djadmin:`loaddata` then :djadmin:`dumpdata`.

Or, if they're small enough, you can edit them to add the UTC offset that

matches your :setting:`TIME_ZONE` to each serialized datetime.

.. _time-zones-faq:

FAQ

===

Setup

-----

#. **I don't need multiple time zones. Should I enable time zone support?**

   Yes. When time zone support is enabled, Django uses a more accurate model

   of local time. This shields you from subtle and unreproducible bugs around

   daylight saving time (DST) transitions.

   When you enable time zone support, you'll encounter some errors because

   you're using naive datetimes where Django expects aware datetimes. Such

   errors show up when running tests. You'll quickly learn how to avoid invalid

   operations.

   On the other hand, bugs caused by the lack of time zone support are much

   harder to prevent, diagnose and fix. Anything that involves scheduled tasks

   or datetime arithmetic is a candidate for subtle bugs that will bite you

   only once or twice a year.

   For these reasons, time zone support is enabled by default in new projects,

   and you should keep it unless you have a very good reason not to.

#. **I've enabled time zone support. Am I safe?**

   Maybe. You're better protected from DST-related bugs, but you can still

   shoot yourself in the foot by carelessly turning naive datetimes into aware

   datetimes, and vice-versa.

   If your application connects to other systems -- for instance, if it queries

   a web service -- make sure datetimes are properly specified. To transmit

   datetimes safely, their representation should include the UTC offset, or

   their values should be in UTC (or both!).

   Finally, our calendar system contains interesting edge cases. For example,

   you can't always subtract one year directly from a given date::

       >>> import datetime

       >>> def one_year_before(value):  # Wrong example.

       ...     return value.replace(year=value.year - 1)

       >>> one_year_before(datetime.datetime(2012, 3, 1, 10, 0))

       datetime.datetime(2011, 3, 1, 10, 0)

       >>> one_year_before(datetime.datetime(2012, 2, 29, 10, 0))

       Traceback (most recent call last):

       ...

       ValueError: day is out of range for month

   To implement such a function correctly, you must decide whether 2012-02-29

   minus one year is 2011-02-28 or 2011-03-01, which depends on your business

   requirements.

#. **How do I interact with a database that stores datetimes in local time?**

   Set the :setting:`TIME_ZONE <DATABASE-TIME_ZONE>` option to the appropriate

   time zone for this database in the :setting:`DATABASES` setting.

   This is useful for connecting to a database that doesn't support time zones

   and that isn't managed by Django when :setting:`USE_TZ` is ``True``.

Troubleshooting

---------------

#. **My application crashes with** ``TypeError: can't compare offset-naive``

   ``and offset-aware datetimes`` **-- what's wrong?**

   Let's reproduce this error by comparing a naive and an aware datetime::

       >>> from django.utils import timezone

       >>> aware = timezone.now()

       >>> naive = timezone.make_naive(aware)

       >>> naive == aware

       Traceback (most recent call last):

       ...

       TypeError: can't compare offset-naive and offset-aware datetimes

   If you encounter this error, most likely your code is comparing these two

   things:

   - a datetime provided by Django -- for instance, a value read from a form or

     a model field. Since you enabled time zone support, it's aware.

   - a datetime generated by your code, which is naive (or you wouldn't be

     reading this).

   Generally, the correct solution is to change your code to use an aware

   datetime instead.

   If you're writing a pluggable application that's expected to work

   independently of the value of :setting:`USE_TZ`, you may find

   :func:`django.utils.timezone.now` useful. This function returns the current

   date and time as a naive datetime when ``USE_TZ = False`` and as an aware

   datetime when ``USE_TZ = True``. You can add or subtract

   :class:`datetime.timedelta` as needed.

#. **I see lots of** ``RuntimeWarning: DateTimeField received a naive

   datetime`` ``(YYYY-MM-DD HH:MM:SS)`` ``while time zone support is active``

   **-- is that bad?**

   When time zone support is enabled, the database layer expects to receive

   only aware datetimes from your code. This warning occurs when it receives a

   naive datetime. This indicates that you haven't finished porting your code

   for time zone support. Please refer to the :ref:`migration guide

   <time-zones-migration-guide>` for tips on this process.

   In the meantime, for backwards compatibility, the datetime is considered to

   be in the default time zone, which is generally what you expect.

#. ``now.date()`` **is yesterday! (or tomorrow)**

   If you've always used naive datetimes, you probably believe that you can

   convert a datetime to a date by calling its :meth:`~datetime.datetime.date`

   method. You also consider that a :class:`~datetime.date` is a lot like a

   :class:`~datetime.datetime`, except that it's less accurate.

   None of this is true in a time zone aware environment::

       >>> import datetime

       >>> import zoneinfo

       >>> paris_tz = zoneinfo.ZoneInfo("Europe/Paris")

       >>> new_york_tz = zoneinfo.ZoneInfo("America/New_York")

       >>> paris = datetime.datetime(2012, 3, 3, 1, 30, tzinfo=paris_tz)

       # This is the correct way to convert between time zones.

       >>> new_york = paris.astimezone(new_york_tz)

       >>> paris == new_york, paris.date() == new_york.date()

       (True, False)

       >>> paris - new_york, paris.date() - new_york.date()

       (datetime.timedelta(0), datetime.timedelta(1))

       >>> paris

       datetime.datetime(2012, 3, 3, 1, 30, tzinfo=zoneinfo.ZoneInfo(key='Europe/Paris'))

       >>> new_york

       datetime.datetime(2012, 3, 2, 19, 30, tzinfo=zoneinfo.ZoneInfo(key='America/New_York'))

   As this example shows, the same datetime has a different date, depending on

   the time zone in which it is represented. But the real problem is more

   fundamental.

   A datetime represents a **point in time**. It's absolute: it doesn't depend

   on anything. On the contrary, a date is a **calendaring concept**. It's a

   period of time whose bounds depend on the time zone in which the date is

   considered. As you can see, these two concepts are fundamentally different,

   and converting a datetime to a date isn't a deterministic operation.

   What does this mean in practice?

   Generally, you should avoid converting a :class:`~datetime.datetime` to

   :class:`~datetime.date`. For instance, you can use the :tfilter:`date`

   template filter to only show the date part of a datetime. This filter will

   convert the datetime into the current time zone before formatting it,

   ensuring the results appear correctly.

   If you really need to do the conversion yourself, you must ensure the

   datetime is converted to the appropriate time zone first. Usually, this

   will be the current timezone::

       >>> from django.utils import timezone

       >>> timezone.activate(zoneinfo.ZoneInfo("Asia/Singapore"))

       # For this example, we set the time zone to Singapore, but here's how

       # you would obtain the current time zone in the general case.

       >>> current_tz = timezone.get_current_timezone()

       >>> local = paris.astimezone(current_tz)

       >>> local

       datetime.datetime(2012, 3, 3, 8, 30, tzinfo=zoneinfo.ZoneInfo(key='Asia/Singapore'))

       >>> local.date()

       datetime.date(2012, 3, 3)

#. **I get an error** "``Are time zone definitions for your database

   installed?``"

   If you are using MySQL, see the :ref:`mysql-time-zone-definitions` section

   of the MySQL notes for instructions on loading time zone definitions.

Usage

-----

#. **I have a string** ``"2012-02-21 10:28:45"`` **and I know it's in the**

   ``"Europe/Helsinki"`` **time zone. How do I turn that into an aware

   datetime?**

   Here you need to create the required ``ZoneInfo`` instance and attach it to

   the naïve datetime::

       >>> import zoneinfo

       >>> from django.utils.dateparse import parse_datetime

       >>> naive = parse_datetime("2012-02-21 10:28:45")

       >>> naive.replace(tzinfo=zoneinfo.ZoneInfo("Europe/Helsinki"))

       datetime.datetime(2012, 2, 21, 10, 28, 45, tzinfo=zoneinfo.ZoneInfo(key='Europe/Helsinki'))

#. **How can I obtain the local time in the current time zone?**

   Well, the first question is, do you really need to?

   You should only use local time when you're interacting with humans, and the

   template layer provides :ref:`filters and tags <time-zones-in-templates>`

   to convert datetimes to the time zone of your choice.

   Furthermore, Python knows how to compare aware datetimes, taking into

   account UTC offsets when necessary. It's much easier (and possibly faster)

   to write all your model and view code in UTC. So, in most circumstances,

   the datetime in UTC returned by :func:`django.utils.timezone.now` will be

   sufficient.

   For the sake of completeness, though, if you really want the local time

   in the current time zone, here's how you can obtain it::

       >>> from django.utils import timezone

       >>> timezone.localtime(timezone.now())

       datetime.datetime(2012, 3, 3, 20, 10, 53, 873365, tzinfo=zoneinfo.ZoneInfo(key='Europe/Paris'))

   In this example, the current time zone is ``"Europe/Paris"``.

#. **How can I see all available time zones?**

   :func:`zoneinfo.available_timezones` provides the set of all valid keys for

   IANA time zones available to your system. See the docs for usage

   considerations.

.. _pytz: http://pytz.sourceforge.net/