686 lines
25 KiB
Plaintext
686 lines
25 KiB
Plaintext
==========
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Time zones
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==========
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.. _time-zones-overview:
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Overview
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========
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When support for time zones is enabled, Django stores datetime information in
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UTC in the database, uses time-zone-aware datetime objects internally, and
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translates them to the end user's time zone in templates and forms.
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This is handy if your users live in more than one time zone and you want to
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display datetime information according to each user's wall clock.
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Even if your website is available in only one time zone, it's still good
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practice to store data in UTC in your database. The main reason is Daylight
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Saving Time (DST). Many countries have a system of DST, where clocks are moved
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forward in spring and backward in autumn. If you're working in local time,
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you're likely to encounter errors twice a year, when the transitions happen.
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(The pytz_ documentation discusses `these issues`_ in greater detail.) This
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probably doesn't matter for your blog, but it's a problem if you over-bill or
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under-bill your customers by one hour, twice a year, every year. The solution
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to this problem is to use UTC in the code and use local time only when
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interacting with end users.
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Time zone support is disabled by default. To enable it, set :setting:`USE_TZ =
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True <USE_TZ>` in your settings file. Time zone support uses pytz_, which is
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installed when you install Django.
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.. note::
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The default :file:`settings.py` file created by :djadmin:`django-admin
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startproject <startproject>` includes :setting:`USE_TZ = True <USE_TZ>`
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for convenience.
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.. note::
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There is also an independent but related :setting:`USE_L10N` setting that
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controls whether Django should activate format localization. See
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:doc:`/topics/i18n/formatting` for more details.
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If you're wrestling with a particular problem, start with the :ref:`time zone
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FAQ <time-zones-faq>`.
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Concepts
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========
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.. _naive_vs_aware_datetimes:
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Naive and aware datetime objects
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--------------------------------
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Python's :class:`datetime.datetime` objects have a ``tzinfo`` attribute that
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can be used to store time zone information, represented as an instance of a
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subclass of :class:`datetime.tzinfo`. When this attribute is set and describes
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an offset, a datetime object is **aware**. Otherwise, it's **naive**.
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You can use :func:`~django.utils.timezone.is_aware` and
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:func:`~django.utils.timezone.is_naive` to determine whether datetimes are
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aware or naive.
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When time zone support is disabled, Django uses naive datetime objects in local
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time. This is simple and sufficient for many use cases. In this mode, to obtain
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the current time, you would write::
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import datetime
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now = datetime.datetime.now()
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When time zone support is enabled (:setting:`USE_TZ=True <USE_TZ>`), Django uses
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time-zone-aware datetime objects. If your code creates datetime objects, they
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should be aware too. In this mode, the example above becomes::
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from django.utils import timezone
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now = timezone.now()
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.. warning::
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Dealing with aware datetime objects isn't always intuitive. For instance,
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the ``tzinfo`` argument of the standard datetime constructor doesn't work
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reliably for time zones with DST. Using UTC is generally safe; if you're
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using other time zones, you should review the `pytz`_ documentation
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carefully.
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.. note::
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Python's :class:`datetime.time` objects also feature a ``tzinfo``
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attribute, and PostgreSQL has a matching ``time with time zone`` type.
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However, as PostgreSQL's docs put it, this type "exhibits properties which
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lead to questionable usefulness".
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Django only supports naive time objects and will raise an exception if you
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attempt to save an aware time object, as a timezone for a time with no
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associated date does not make sense.
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.. _naive-datetime-objects:
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Interpretation of naive datetime objects
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----------------------------------------
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When :setting:`USE_TZ` is ``True``, Django still accepts naive datetime
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objects, in order to preserve backwards-compatibility. When the database layer
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receives one, it attempts to make it aware by interpreting it in the
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:ref:`default time zone <default-current-time-zone>` and raises a warning.
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Unfortunately, during DST transitions, some datetimes don't exist or are
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ambiguous. In such situations, pytz_ raises an exception. That's why you should
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always create aware datetime objects when time zone support is enabled.
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In practice, this is rarely an issue. Django gives you aware datetime objects
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in the models and forms, and most often, new datetime objects are created from
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existing ones through :class:`~datetime.timedelta` arithmetic. The only
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datetime that's often created in application code is the current time, and
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:func:`timezone.now() <django.utils.timezone.now>` automatically does the
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right thing.
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.. _default-current-time-zone:
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Default time zone and current time zone
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---------------------------------------
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The **default time zone** is the time zone defined by the :setting:`TIME_ZONE`
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setting.
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The **current time zone** is the time zone that's used for rendering.
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You should set the current time zone to the end user's actual time zone with
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:func:`~django.utils.timezone.activate`. Otherwise, the default time zone is
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used.
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.. note::
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As explained in the documentation of :setting:`TIME_ZONE`, Django sets
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environment variables so that its process runs in the default time zone.
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This happens regardless of the value of :setting:`USE_TZ` and of the
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current time zone.
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When :setting:`USE_TZ` is ``True``, this is useful to preserve
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backwards-compatibility with applications that still rely on local time.
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However, :ref:`as explained above <naive-datetime-objects>`, this isn't
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entirely reliable, and you should always work with aware datetimes in UTC
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in your own code. For instance, use :meth:`~datetime.datetime.fromtimestamp`
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and set the ``tz`` parameter to :data:`~django.utils.timezone.utc`.
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Selecting the current time zone
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-------------------------------
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The current time zone is the equivalent of the current :term:`locale <locale
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name>` for translations. However, there's no equivalent of the
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``Accept-Language`` HTTP header that Django could use to determine the user's
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time zone automatically. Instead, Django provides :ref:`time zone selection
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functions <time-zone-selection-functions>`. Use them to build the time zone
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selection logic that makes sense for you.
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Most websites that care about time zones just ask users in which time zone they
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live and store this information in the user's profile. For anonymous users,
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they use the time zone of their primary audience or UTC. pytz_ provides
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helpers_, like a list of time zones per country, that you can use to pre-select
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the most likely choices.
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Here's an example that stores the current timezone in the session. (It skips
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error handling entirely for the sake of simplicity.)
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Add the following middleware to :setting:`MIDDLEWARE`::
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import pytz
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from django.utils import timezone
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from django.utils.deprecation import MiddlewareMixin
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class TimezoneMiddleware(MiddlewareMixin):
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def process_request(self, request):
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tzname = request.session.get('django_timezone')
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if tzname:
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timezone.activate(pytz.timezone(tzname))
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else:
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timezone.deactivate()
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Create a view that can set the current timezone::
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from django.shortcuts import redirect, render
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def set_timezone(request):
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if request.method == 'POST':
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request.session['django_timezone'] = request.POST['timezone']
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return redirect('/')
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else:
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return render(request, 'template.html', {'timezones': pytz.common_timezones})
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Include a form in ``template.html`` that will ``POST`` to this view:
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.. code-block:: html+django
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{% load tz %}
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{% get_current_timezone as TIME_ZONE %}
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<form action="{% url 'set_timezone' %}" method="POST">
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{% csrf_token %}
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<label for="timezone">Time zone:</label>
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<select name="timezone">
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{% for tz in timezones %}
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<option value="{{ tz }}"{% if tz == TIME_ZONE %} selected{% endif %}>{{ tz }}</option>
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{% endfor %}
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</select>
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<input type="submit" value="Set">
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</form>
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.. _time-zones-in-forms:
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Time zone aware input in forms
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==============================
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When you enable time zone support, Django interprets datetimes entered in
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forms in the :ref:`current time zone <default-current-time-zone>` and returns
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aware datetime objects in ``cleaned_data``.
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If the current time zone raises an exception for datetimes that don't exist or
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are ambiguous because they fall in a DST transition (the timezones provided by
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pytz_ do this), such datetimes will be reported as invalid values.
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.. _time-zones-in-templates:
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Time zone aware output in templates
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===================================
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When you enable time zone support, Django converts aware datetime objects to
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the :ref:`current time zone <default-current-time-zone>` when they're rendered
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in templates. This behaves very much like :doc:`format localization
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</topics/i18n/formatting>`.
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.. warning::
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Django doesn't convert naive datetime objects, because they could be
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ambiguous, and because your code should never produce naive datetimes when
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time zone support is enabled. However, you can force conversion with the
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template filters described below.
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Conversion to local time isn't always appropriate -- you may be generating
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output for computers rather than for humans. The following filters and tags,
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provided by the ``tz`` template tag library, allow you to control the time zone
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conversions.
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.. highlight:: html+django
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Template tags
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-------------
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.. templatetag:: localtime
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``localtime``
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~~~~~~~~~~~~~
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Enables or disables conversion of aware datetime objects to the current time
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zone in the contained block.
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This tag has exactly the same effects as the :setting:`USE_TZ` setting as far
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as the template engine is concerned. It allows a more fine grained control of
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conversion.
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To activate or deactivate conversion for a template block, use::
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{% load tz %}
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{% localtime on %}
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{{ value }}
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{% endlocaltime %}
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{% localtime off %}
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{{ value }}
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{% endlocaltime %}
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.. note::
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The value of :setting:`USE_TZ` isn't respected inside of a
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``{% localtime %}`` block.
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.. templatetag:: timezone
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``timezone``
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~~~~~~~~~~~~
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Sets or unsets the current time zone in the contained block. When the current
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time zone is unset, the default time zone applies.
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::
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{% load tz %}
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{% timezone "Europe/Paris" %}
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Paris time: {{ value }}
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{% endtimezone %}
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{% timezone None %}
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Server time: {{ value }}
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{% endtimezone %}
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.. templatetag:: get_current_timezone
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``get_current_timezone``
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~~~~~~~~~~~~~~~~~~~~~~~~
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You can get the name of the current time zone using the
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``get_current_timezone`` tag::
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{% get_current_timezone as TIME_ZONE %}
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Alternatively, you can activate the
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:func:`~django.template.context_processors.tz` context processor and
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use the ``TIME_ZONE`` context variable.
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Template filters
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----------------
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These filters accept both aware and naive datetimes. For conversion purposes,
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they assume that naive datetimes are in the default time zone. They always
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return aware datetimes.
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.. templatefilter:: localtime
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``localtime``
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~~~~~~~~~~~~~
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Forces conversion of a single value to the current time zone.
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For example::
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{% load tz %}
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{{ value|localtime }}
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.. templatefilter:: utc
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``utc``
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~~~~~~~
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Forces conversion of a single value to UTC.
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For example::
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{% load tz %}
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{{ value|utc }}
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.. templatefilter:: timezone
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``timezone``
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~~~~~~~~~~~~
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Forces conversion of a single value to an arbitrary timezone.
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The argument must be an instance of a :class:`~datetime.tzinfo` subclass or a
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time zone name.
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For example::
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{% load tz %}
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{{ value|timezone:"Europe/Paris" }}
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.. highlight:: python
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.. _time-zones-migration-guide:
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Migration guide
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===============
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Here's how to migrate a project that was started before Django supported time
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zones.
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Database
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--------
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PostgreSQL
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~~~~~~~~~~
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The PostgreSQL backend stores datetimes as ``timestamp with time zone``. In
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practice, this means it converts datetimes from the connection's time zone to
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UTC on storage, and from UTC to the connection's time zone on retrieval.
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As a consequence, if you're using PostgreSQL, you can switch between ``USE_TZ
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= False`` and ``USE_TZ = True`` freely. The database connection's time zone
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will be set to :setting:`TIME_ZONE` or ``UTC`` respectively, so that Django
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obtains correct datetimes in all cases. You don't need to perform any data
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conversions.
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Other databases
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~~~~~~~~~~~~~~~
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Other backends store datetimes without time zone information. If you switch
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from ``USE_TZ = False`` to ``USE_TZ = True``, you must convert your data from
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local time to UTC -- which isn't deterministic if your local time has DST.
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Code
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----
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The first step is to add :setting:`USE_TZ = True <USE_TZ>` to your settings
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file. At this point, things should mostly work. If you create naive datetime
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objects in your code, Django makes them aware when necessary.
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However, these conversions may fail around DST transitions, which means you
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aren't getting the full benefits of time zone support yet. Also, you're likely
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to run into a few problems because it's impossible to compare a naive datetime
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with an aware datetime. Since Django now gives you aware datetimes, you'll get
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exceptions wherever you compare a datetime that comes from a model or a form
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with a naive datetime that you've created in your code.
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So the second step is to refactor your code wherever you instantiate datetime
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objects to make them aware. This can be done incrementally.
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:mod:`django.utils.timezone` defines some handy helpers for compatibility
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code: :func:`~django.utils.timezone.now`,
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:func:`~django.utils.timezone.is_aware`,
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:func:`~django.utils.timezone.is_naive`,
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:func:`~django.utils.timezone.make_aware`, and
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:func:`~django.utils.timezone.make_naive`.
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Finally, in order to help you locate code that needs upgrading, Django raises
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a warning when you attempt to save a naive datetime to the database::
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RuntimeWarning: DateTimeField ModelName.field_name received a naive
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datetime (2012-01-01 00:00:00) while time zone support is active.
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During development, you can turn such warnings into exceptions and get a
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traceback by adding the following to your settings file::
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import warnings
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warnings.filterwarnings(
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'error', r"DateTimeField .* received a naive datetime",
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RuntimeWarning, r'django\.db\.models\.fields',
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)
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Fixtures
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--------
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When serializing an aware datetime, the UTC offset is included, like this::
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"2011-09-01T13:20:30+03:00"
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For a naive datetime, it obviously isn't::
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"2011-09-01T13:20:30"
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For models with :class:`~django.db.models.DateTimeField`\ s, this difference
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makes it impossible to write a fixture that works both with and without time
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zone support.
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Fixtures generated with ``USE_TZ = False``, or before Django 1.4, use the
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"naive" format. If your project contains such fixtures, after you enable time
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zone support, you'll see :exc:`RuntimeWarning`\ s when you load them. To get
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rid of the warnings, you must convert your fixtures to the "aware" format.
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You can regenerate fixtures with :djadmin:`loaddata` then :djadmin:`dumpdata`.
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Or, if they're small enough, you can simply edit them to add the UTC offset
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that matches your :setting:`TIME_ZONE` to each serialized datetime.
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.. _time-zones-faq:
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FAQ
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===
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Setup
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-----
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1. **I don't need multiple time zones. Should I enable time zone support?**
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Yes. When time zone support is enabled, Django uses a more accurate model
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of local time. This shields you from subtle and unreproducible bugs around
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Daylight Saving Time (DST) transitions.
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When you enable time zone support, you'll encounter some errors because
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you're using naive datetimes where Django expects aware datetimes. Such
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errors show up when running tests and they're easy to fix. You'll quickly
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learn how to avoid invalid operations.
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On the other hand, bugs caused by the lack of time zone support are much
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harder to prevent, diagnose and fix. Anything that involves scheduled tasks
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or datetime arithmetic is a candidate for subtle bugs that will bite you
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only once or twice a year.
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For these reasons, time zone support is enabled by default in new projects,
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and you should keep it unless you have a very good reason not to.
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2. **I've enabled time zone support. Am I safe?**
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Maybe. You're better protected from DST-related bugs, but you can still
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shoot yourself in the foot by carelessly turning naive datetimes into aware
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datetimes, and vice-versa.
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If your application connects to other systems -- for instance, if it queries
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a Web service -- make sure datetimes are properly specified. To transmit
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datetimes safely, their representation should include the UTC offset, or
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their values should be in UTC (or both!).
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Finally, our calendar system contains interesting traps for computers::
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>>> import datetime
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>>> def one_year_before(value): # DON'T DO THAT!
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... return value.replace(year=value.year - 1)
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>>> one_year_before(datetime.datetime(2012, 3, 1, 10, 0))
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datetime.datetime(2011, 3, 1, 10, 0)
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>>> one_year_before(datetime.datetime(2012, 2, 29, 10, 0))
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Traceback (most recent call last):
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...
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ValueError: day is out of range for month
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(To implement this function, you must decide whether 2012-02-29 minus
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one year is 2011-02-28 or 2011-03-01, which depends on your business
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requirements.)
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3. **How do I interact with a database that stores datetimes in local time?**
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Set the :setting:`TIME_ZONE <DATABASE-TIME_ZONE>` option to the appropriate
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time zone for this database in the :setting:`DATABASES` setting.
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This is useful for connecting to a database that doesn't support time zones
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and that isn't managed by Django when :setting:`USE_TZ` is ``True``.
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Troubleshooting
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---------------
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1. **My application crashes with** ``TypeError: can't compare offset-naive``
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``and offset-aware datetimes`` **-- what's wrong?**
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Let's reproduce this error by comparing a naive and an aware datetime::
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>>> import datetime
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>>> from django.utils import timezone
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>>> naive = datetime.datetime.utcnow()
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>>> aware = timezone.now()
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>>> naive == aware
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Traceback (most recent call last):
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...
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TypeError: can't compare offset-naive and offset-aware datetimes
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If you encounter this error, most likely your code is comparing these two
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things:
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- a datetime provided by Django -- for instance, a value read from a form or
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a model field. Since you enabled time zone support, it's aware.
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- a datetime generated by your code, which is naive (or you wouldn't be
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reading this).
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Generally, the correct solution is to change your code to use an aware
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datetime instead.
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If you're writing a pluggable application that's expected to work
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independently of the value of :setting:`USE_TZ`, you may find
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:func:`django.utils.timezone.now` useful. This function returns the current
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date and time as a naive datetime when ``USE_TZ = False`` and as an aware
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datetime when ``USE_TZ = True``. You can add or subtract
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:class:`datetime.timedelta` as needed.
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2. **I see lots of** ``RuntimeWarning: DateTimeField received a naive
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datetime`` ``(YYYY-MM-DD HH:MM:SS)`` ``while time zone support is active``
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**-- is that bad?**
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|
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When time zone support is enabled, the database layer expects to receive
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only aware datetimes from your code. This warning occurs when it receives a
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naive datetime. This indicates that you haven't finished porting your code
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for time zone support. Please refer to the :ref:`migration guide
|
|
<time-zones-migration-guide>` for tips on this process.
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In the meantime, for backwards compatibility, the datetime is considered to
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be in the default time zone, which is generally what you expect.
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3. ``now.date()`` **is yesterday! (or tomorrow)**
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If you've always used naive datetimes, you probably believe that you can
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convert a datetime to a date by calling its :meth:`~datetime.datetime.date`
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method. You also consider that a :class:`~datetime.date` is a lot like a
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:class:`~datetime.datetime`, except that it's less accurate.
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|
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None of this is true in a time zone aware environment::
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|
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>>> import datetime
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>>> import pytz
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>>> paris_tz = pytz.timezone("Europe/Paris")
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>>> new_york_tz = pytz.timezone("America/New_York")
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>>> paris = paris_tz.localize(datetime.datetime(2012, 3, 3, 1, 30))
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# This is the correct way to convert between time zones with pytz.
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>>> new_york = new_york_tz.normalize(paris.astimezone(new_york_tz))
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>>> paris == new_york, paris.date() == new_york.date()
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(True, False)
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>>> paris - new_york, paris.date() - new_york.date()
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(datetime.timedelta(0), datetime.timedelta(1))
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>>> paris
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datetime.datetime(2012, 3, 3, 1, 30, tzinfo=<DstTzInfo 'Europe/Paris' CET+1:00:00 STD>)
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>>> new_york
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datetime.datetime(2012, 3, 2, 19, 30, tzinfo=<DstTzInfo 'America/New_York' EST-1 day, 19:00:00 STD>)
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|
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|
As this example shows, the same datetime has a different date, depending on
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|
the time zone in which it is represented. But the real problem is more
|
|
fundamental.
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|
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|
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.
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|
|
|
What does this mean in practice?
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|
|
|
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)
|
|
|
|
4. **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
|
|
-----
|
|
|
|
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, 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: https://en.wikipedia.org/wiki/Tz_database
|