lib/django-1.5/docs/topics/i18n/timezones.txt

.. _time-zones:

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

.. versionadded:: 1.4

.. _time-zones-overview:

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 Web site is available in only one time zone, it's still good
practice to store data in UTC in your database. One 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.
(The pytz_ documentation discusses `these issues`_ in greater detail.) 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. Installing pytz_ is highly recommended,
but not mandatory. It's as simple as:

.. code-block:: bash

    $ sudo pip install pytz

.. 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 whether Django should activate format localization. See
    :doc:`/topics/i18n/formatting` for more details.

If you're wrestling with a particular problem, start with the :ref:`time zone
FAQ <time-zones-faq>`.

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 whether 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 the `pytz`_ 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.

.. _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.

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.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 Web sites that 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['django_timezone'] = pytz.timezone(request.POST['timezone'])
            return redirect('/')
        else:
            return render(request, 'template.html', {'timezones': pytz.common_timezones})

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

.. code-block:: html+django

    {% load tz %}
    <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-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``.

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 by the ``tz`` template tag 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 %}

.. templatetag:: get_current_timezone

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

When the ``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.

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 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 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 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"

For a naive datetime, it obviously 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:`~exceptions.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 simply edit them to add the UTC offset
that matches your :setting:`TIME_ZONE` to each serialized datetime.

.. _time-zones-faq:

FAQ
===

Setup
-----

1. **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.

   In this regard, time zones are comparable to ``unicode`` in Python. At first
   it's hard. You get encoding and decoding errors. Then you learn the rules.
   And some problems disappear -- you never get mangled output again when your
   application receives non-ASCII input.

   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 and they're easy to fix. 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.

2. **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 traps for computers::

       >>> import datetime
       >>> def one_year_before(value):       # DON'T DO THAT!
       ...     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 this function, you must decide whether 2012-02-29 minus
   one year is 2011-02-28 or 2011-03-01, which depends on your business
   requirements.)

3. **Should I install pytz?**

   Yes. Django has a policy of not requiring external dependencies, and for
   this reason pytz_ is optional. However, it's much safer to install it.

   As soon as you activate time zone support, Django needs a definition of the
   default time zone. When pytz is available, Django loads this definition
   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.

   Furthermore, if you want to support users in more than one time zone, pytz
   is the reference for time zone definitions.

Troubleshooting
---------------

1. **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::

       >>> import datetime
       >>> from django.utils import timezone
       >>> naive = datetime.datetime.utcnow()
       >>> aware = naive.replace(tzinfo=timezone.utc)
       >>> 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.

2. **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.

3. ``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 pytz
       >>> paris_tz = pytz.timezone("Europe/Paris")
       >>> new_york_tz = pytz.timezone("America/New_York")
       >>> paris = paris_tz.localize(datetime.datetime(2012, 3, 3, 1, 30))
       # This is the correct way to convert between time zones with pytz.
       >>> new_york = new_york_tz.normalize(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=<DstTzInfo 'Europe/Paris' CET+1:00:00 STD>)
       >>> new_york
       datetime.datetime(2012, 3, 2, 19, 30, tzinfo=<DstTzInfo 'America/New_York' EST-1 day, 19:00:00 STD>)

   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(pytz.timezone("Asia/Singapore"))
       # For this example, we just 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()
       # Again, this is the correct way to convert between time zones with pytz.
       >>> local = current_tz.normalize(paris.astimezone(current_tz))
       >>> local
       datetime.datetime(2012, 3, 3, 8, 30, tzinfo=<DstTzInfo 'Asia/Singapore' SGT+8:00:00 STD>)
       >>> local.date()
       datetime.date(2012, 3, 3)

Usage
-----

1. **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?**

   This is exactly what pytz_ is for.

       >>> from django.utils.dateparse import parse_datetime
       >>> naive = parse_datetime("2012-02-21 10:28:45")
       >>> import pytz
       >>> pytz.timezone("Europe/Helsinki").localize(naive, is_dst=None)
       datetime.datetime(2012, 2, 21, 10, 28, 45, tzinfo=<DstTzInfo 'Europe/Helsinki' EET+2:00:00 STD>)

   Note that ``localize`` is a pytz extension to the :class:`~datetime.tzinfo`
   API. Also, you may want to catch ``pytz.InvalidTimeError``. The
   documentation of pytz contains `more examples`_. You should review it
   before attempting to manipulate aware datetimes.

2. **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=<DstTzInfo 'Europe/Paris' CET+1:00:00 STD>)

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

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

   pytz_ provides helpers_, including a list of current time zones and a list
   of all available time zones -- some of which are only of historical
   interest.

.. _pytz: http://pytz.sourceforge.net/
.. _more examples: http://pytz.sourceforge.net/#example-usage
.. _these issues: http://pytz.sourceforge.net/#problems-with-localtime
.. _helpers: http://pytz.sourceforge.net/#helpers
.. _tz database: http://en.wikipedia.org/wiki/Tz_database