Time zones¶

Naive and aware datetime objects¶

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

You can use is_aware() and 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)

Interpretation of naive datetime objects¶

When 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 default 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 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 timedelta arithmetic. The only datetime that’s often created in application code is the current time, and timezone.now() automatically does the right thing.

Default time zone and current time zone¶

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

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

You should set the current time zone to the end user’s actual time zone with activate(). Otherwise, the default time zone is used.

Selecting the current time zone¶

The current time zone is the equivalent of the current locale 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 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 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:

{% 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>

Template tags¶

{% load tz %}

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

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

{% load tz %}

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

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

{% 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.

{% load tz %}

{{ value|localtime }}

{% load tz %}

{{ value|utc }}

{% load tz %}

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

Database¶

Code¶

The first step is to add USE_TZ = True 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. django.utils.timezone defines some handy helpers for compatibility code: now(), is_aware(), is_naive(), make_aware(), and 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 DateTimeFields, 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 RuntimeWarnings when you load them. To get rid of the warnings, you must convert your fixtures to the “aware” format.

You can regenerate fixtures with loaddata then dumpdata. Or, if they’re small enough, you can simply edit them to add the UTC offset that matches your TIME_ZONE to each serialized datetime.

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
   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
   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 USE_TZ, you may find 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 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 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 date() method. You also consider that a date is a lot like a 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 datetime to date. For instance, you can use the 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 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 filters and tags 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 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.