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Timezone in Websites • Totally broken in all possible ways • Two negatives make a positive! • But if you (by chance) get some of them right… 

Databases • All databases do not support timezones • PostgreSQL: yes • SQLite: no • MySQL: sometimes • Django stores everything in UTC 

Naïve vs Aware >>> import datetime! >>> now = datetime.datetime.now()! >>> print now! 2014-01-21 19:09:14.960786! >>> print now.replace(tzinfo=taipei)! 2014-01-21 19:09:14.960786+08:00 

Naïve vs Aware >>> import datetime! >>> now = datetime.datetime.now()! >>> print now! 2014-01-21 19:09:14.960786! >>> print now.replace(tzinfo=taipei)! 2014-01-21 19:09:14.960786+08:00 naïve aware 

naïve /nai-eev/ 1. Lacking worldly experience, wisdom, or judgement; unsophisticated. Surely you're not naïve enough to believe adverts! … http://en.wiktionary.org/wiki/naive 

pytz • The Olson Timezone Database • pip install pytz • taipei = pytz.timezone(‘Asia/Taipei') • Django depends on this for timezone support http://pytz.sourceforge.net 

USE_TZ • USE_TZ = False • Naïve objects with your default timezone • USE_TZ = True • Aware objects (Naïve ones generate warnings) • Django makes a date/time aware automatically when it’s retrieved from database 

>>> from django.utils.timezone import *! >>> print now()! 2014-01-21 11:09:14.960786+00:00! >>> from django.conf import settings! >>> print settings.USE_TZ! True django.utils.timezone 

HTTP • HTTP header doesn’t have a “Accept-Timezone” field • You need to manually set a request’s timezone if you need timezone support 

from django.utils.timezone import *! from pytz import timezone! ! taipei = timezone('Asia/Taipei')! activate(taipei)! ! print get_current_timezone_name()! # Asia/Tapei! ! with override(utc):! print get_current_timezone_name()! # UTC! ! deactivete() 

{# "value" is a date/time object #}! ! {% load tz %}! ! {# Neither of these respect USE_TZ #}! {% localtime on %}! {{ value }}! {% endlocaltime %}! ! {% localtime off %}! {{ value }}! {% endlocaltime %} 

{% load tz %}! ! {% timezone "Europe/Paris" %}! Paris time: {{ value }}! {% endtimezone %}! ! {% get_current_timezone as timezone %}! ! {{ value|localtime }}! ! {{ value|utc }}! ! {{ value|timezone:"Europe/Paris" }} 

DST • No universal standard • Varies every year • Some nations only partly use it (e.g. Australia) • pytz helps (but you still need to be cautious) 

Modern Technology • HTML5 Geolocation API • Client-side processing • Still not reliable 

Related Reading http://docs.python.org/2/library/datetime.html http://pytz.sourceforge.net https://docs.djangoproject.com/en/1.6/topics/i18n/timezones/ http://d.pr/S9UL (Datetime, timetuple and timestamp) https://gist.github.com/moskytw/7818553