We Keep Coding

How should I handle datetimes with Django and PosgeSQL?

I have a Django app which needs to support different time zones. I'm saving events to my database which have timestamps and based on those timestamps I want to send notifications. Because the user can move into different time zone, I want to store timestamps as UTC time and convert them to user current time zone later. If I'm correct, PostgreSQL converts datetimes to UTC automatically? At the moment I get an error RuntimeWarning: DateTimeField event.timestamp received a naive datetime (2022-02-15 15:00:00) while time zone support is active.. I used to use Django's make_aware when saving the timestamp and didn't get this warning then. The questions I'd like to get an answer are:
Should I save aware datetimes to the database if I want to support multiple time zones?
What should I use in variables TIME_ZONE and USE_TZ in settings.py? UTC and True?
I store the time zone info to the database, what is the correct method to activate it when I want to get the local time for the user? django.utils.timezone.activate + get_current_timezone or is there some function like astimezone(string_from_db)?

Should I save aware datetimes to the database if I want to support multiple time zones?
The thing that helped me understand timezones is the fact, that time when presented as unix timestamp is a constant and does not depend on where the user is on the globe. Timestamp is saved into Postgres as such. Timezone is just an optional extra information, nothing more. One can take unix timestamp number and transform it into Timezone A, B or whatever he wishes. This depends totally on the request.
Storing timezone information to the database only says that you have to provide timezone when persisting model attribute, otherwise you will get that warning. Warning means that database doesn't know how to treat it and will consider it to be in default timezone, which is usually wrong.
I find it very convenient to store timezone information into the database, but not because it "supports multiple time zones", but because it is less confusing and cleaner. Your users are already in multiple time zones and they are all expecting to see time in their own timezone. If you use timezones, then you don't need to convert to and from UTC and ponder if this number is already converted or not.
I faced issues such as: when a user creates an item, after a single second he sees that an item was created 2 hours ago because you did not include timezone in creation time.
What should I use in variables?
USE_TZ = True
TIME_ZONE = "UTC"
In Django 5.0 the USE_TZ will become True by default.
what is the correct method to get the local time for the user
If you return time values with timezone, the JavaScript easily transforms them to user's timezone by default. If you are using Django templates and all times should appear in different timezone than the user is in, then it is a bit more involved: need to save user's preferred timezone in his profile and activate it with timezone.activate as you correctly pointed out.

PostgreSQL SQLalchemy and timestamp with time zone, why is incognito different from normal?

I am using flask sqlalchemy and postgreSQL and I have issues with the displayed datetimes, while investigating this issue I found an other weird thing:
Creating a DB entry in incognito mode (chrome browser tab) gives a different/wrong time.
EDIT: It has nothing to do with incognito mode, both cases happen in normal mode aswell. I have not figured out yet why.
This is the code:
I changed the default time zone of my database:
ALTER DATABASE postgres SET timezone TO 'Europe/Berlin';
the model:
class User(UserMixin, Base):
__tablename__ = 'users'
date_added = Column(DateTime(timezone=True), nullable=False)
The method I use to add datetime to DB:
date_added=datetime.today()
How it looks in the DB (my localtime at this time was 13:53:46):
Creating entry not in incognito
timestamp with time zone
2019-02-01 13:53:46.73817+01
Creating entry in incognito
timestamp with time zone
2019-02-01 12:53:46.73817+01
This really makes me worry. It is simply wrong. Even if I would convert the datetime objects to localtime. Both entries have been done at the same time, but show different results, how is this possible?
Also when viewing these dates in HTML, postgreSQL does not apply the offset, so the first date looks correct, but the second is wrong.
Initially I just wanted to find a way to store ALL datetime objects in Europe/Berlin and return them in Europe/Berlin time, so I dont have to convert UTC to Europe/Berlin, but now I think something went horribly wrong.
I also have double checked my code everywhere, I am not using other methods to manipulate datetime objects.
EDIT
I am saving a datetime everytime a user logs in. Currently I tried this on not incognito. My localtime is 14:13:33 BUT it saved into the DB:
2019-02-01 13:13:33.804339+01. How is this even possible? I know it cant be random, but right now it looks like its saving times randomly sometimes UTC with offset and sometimes Europe/Berlin with offset.
EDIT
I double checked all tables in question with SHOW timezone; and they all correctly return Europe/Berlin

datetime.today() returns a timestamp without timezone info of the current local time (the returned value is timezone naive). The root of your problem is that somewhere between SQL Alchemy's postgres adapter and postgres itself it has to guess at a timezone. As you might imagine, computer systems tend toward assuming UTC if no timezone is explicitly provided, but the precise logic for a toolsuite set can be complex and hard to debug (and my depend on your local time zone settings on your computer, system-level settings in the db, session level settings, and the preference of the toolmaker). You can sidestep this entire can of worms by either:
Storing all of your timestamps without a time zone in UTC, then converting to the desired timestamp as appropriate
Always using timestamps with time zone (i.e. replace datetime.today() with datetime.now() and pass in the desired time zone so that you are always dealing with a timezone aware value ), so there is no need for the computer to assume a timezone.
Note that in postgres, the timestamp with time zone type is still always stored as UTC with no extra information, the database simply uses the session-level configuration to decide which time zone to display it as when outputting it.

Datetime value on openerp and postgreSQL

Anyone know how to show same data in a datetime field in openerp 6.1 and postgresql?
When I create data at 7:00 (My timezone is +8:00) the view will show me that the data is 7/8/2015 7:00:00 but when I checked the database it became 6/8/2015 22:00:00
Even when I use
date : fields.date.context_today,
Any idea so the view and database datetime data will show the same result?

Times are saved in UTC so that Odoo/OpenERP can support multiple Timezone within a single database. As such all the code related to datetime (at server's level and web UI level) has been build with that assumption in mind. If you need to get the value in different timezone from the database, I believe it is for easier to calculate it from the field's value rather than getting Odoo/OpenERP to manage (storing is just only part of it) the time value in non-UTC timezone.

Django Timezone UTC

I am currently working on Django project with a postgres DB.
The data stored in db with time-stamp using Naive time (user's local time).
However, in the setting.py, we have
USE_TZ = True
which means all the time-stamps retrieved by Django ORM are converted to UTC.
Generally, it is ok. However, for the function I am building right now does need the real time (uesr's local time). Certainly, I would fetch the data and convert the time to whatever time I want, with two problems:
1. I can convert time-stamps to EST or anything, but I still don't know the original time;
2. I want to do the converting during the ORM query rather than after, since it will be more efficient.
Does anyone have a clue about this?
Thanks in advance!

1) If the original datetimes really are naive then I would assume they got stored as whatever timezone you have set in your TIME_ZONE setting (defaults to 'America/Chicago' but might be something else in your case). So converting back to that timezone will probably give you the original time. From the django docs:
"When USE_TZ is False, this is the time zone in which Django will
store all datetimes. When USE_TZ is True, this is the default time
zone that Django will use to display datetimes in templates and to
interpret datetimes entered in forms."
(https://docs.djangoproject.com/en/1.6/ref/settings/#std:setting-TIME_ZONE)
Normally the timezone information is actually set on the db connection (https://docs.djangoproject.com/en/dev/ref/databases/#optimizing-postgresql-s-configuration), so it's expected that you don't get your datetime in UTC when connecting to postgres through psql since the timezone used will default to your system's timezone, I'm not sure why this doesn't happen with raw queries in django though.
2) I can't say I've done many hours of research, however I believe that setting USE_TZ kind of dooms you to have to convert post-query. You can possibly override the connection timezone, but I don't know of an easy way to do this at runtime since it will default to UTC due to USE_TZ.
On previous projects I've worked on and ran into similar issues we have either passed the responsibility over to the front-end as someone suggested in your comments(frontend returns UTC datetimes and converts data back from UTC) or we also store the user timezone and do a post-query conversion. This didn't prove to be inefficient in our use case.

django, datetime and timezones

I am using: datetime.now() to get the current time in an Event app that lets you create an event that has an end date, then all of the events are displayed in a calendar and if an event is passed due it is displayed in red.
My issue is that I have some users in different timezones than me saying that the events are ending at the wrong time. They should end at midnight on the day they are due.
I have the timezone setup in my django settings.py. When I use: datetime.now() is that going off of the users local timezone or is it going off of what timezone I have setup in django?
What I want is to find midnight for the users current timezone, so if my method above is wrong, how do I go about doing that?
Thanks

You will need your users to specify their timezone in their user profile. This can then be used to calculate local times correctly.
Check out Relativity of time – shortcomings in Python datetime, and workaround for some good information (and concrete examples).
UPDATE: From Django 1.4 it comes up with timezone support. Check it out.

I worked on a library to work with user timezones transparently. You should just use a Field and some timezone utils and you should get everything converted to the user timezone everytime you do a request or a query.
The library is named django-timezones, and is a modification of the one that Brosner first made.
Give it a try to see if it works for you.

Try storing the UTC Datetime instead then make the necessary adjustments based on the user's timezone:
import datetime
def getUtcNow():
return datetime.datetime(*time.gmtime()[:6])