Navigating python datetime

If a given datetime is in UTC, will it still be the same if we convert it to central timezone (America/Chicago)?

Background

This week I was tackling a bug that involved datetime and timezones. After working on it for a while I needed help. Not necessarily a hard problem but timezones are confusing and can quickly make the problem worse.

Luckily at my work, Octopus Energy, we've great team members that are always there to help. And David Seddon offerred to help.

David, if you're reading this blog, Thank you 🙏

You should check out his blog, he has some intereseting topics there.

Answer

Yes it will be. In David's words (paraphrasing):

Two datetimes will be equal to each other if they are in the same moment.

That threw me off 🤯

But after spending some time and really undestanding what he meant, it finally clicked.

UTC: Is a time standard by which we represent a particular instance of time globally. So basically if a timezone let's say "X" is represented in UTC, it means "X" is expressed in a positive or negative offset from UTC (from wikipedia).

For example:

CENTRAL_TIMEZONE = pytz.timezone("America/Chicago")

def compare_utc_and_central() -> bool:
    central_datetime = datetime(2021, 10, 5, tzinfo=CENTRAL_TIMEZONE)
    utc_datetime = central_datetime.astimezone(timezone.utc)

    print(central_datetime) # prints: 2021-10-05 00:00:00-05:51
    print(utc_datetime) # prints: 2021-10-05 05:51:00+00:00

    return central_datetime == utc_datetime # Returns True

Another example: Let's say you have a UTC datetime and you want to convert it into a central timezone, but this time without calling the astimezone. Also useful if you've tests and want to assert at specific time.

import freezegun
import pytz

from datetime import date, datetime, timezone
from typing import Tuple


def get_central_datetime_using_utc_datetime(utc_datetime:datetime) -> bool:
    """
    Notice the usage of freezegun. We have to make sure that we are in the same moment
    and freezegun allows us to do that.
    """
    with freezegun.freeze_time(utc_datetime):
        central_datetime = datetime.now(tz=CENTRAL_TIMEZONE)

    return central_datetime == utc_datetime # Returns True

In summary, this was a great learning experience for me. And because of David, I can say that I somewhat have an understanding of timezones in python. Not all obviously, hence the title of this blog is Part 1 of many. There's still a lot to learn.