How should I handle datetimes with Django and PosgeSQL?

Question

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)?

Answer 1

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.