I had a timezone bug... again

Rebeca Sarai · April 6, 2019

Originally published on medium

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When you work with time in a software product, timezone bugs will appear once in a while. Sometimes you forget to consider a DST change, sometimes you consider it wrong, and so on. I realized that even though the same error doesn’t happen twice, the previously solved errors give the foundation to easily identify new ones. So, to not keep trusting my memory here’s the history of my timezone bug.

The application structure

  • Python in the form of Django framework, as your backend
  • Postgres, as your database
  • And some javascript framework as your frontend.

The frontend is not important here, all the bug context was on the backend.

The general scenario

Let’s assume we have a User abstraction— like the following model — and we wanna show on a graph the number of new members per day on the last month.

from django.db import models
class User():
    email = models.EmailField()
    member_since = models.DateTimeField()
    first_name = models.CharField(max_length=255)
    last_name = models.CharField(max_length=255)

Simple right?

The bug scenario

To pull the data and fill in the graph we made a query using Django ORM.

# start_date = '2019-01-01'
# end_date = '2019-02-01'

Quick explaining, this query filters all users with the field member_since between the start_date and the end_date, after that creates a new field called date with only the date information, ignoring all hour and timezone information then extracts all the dates to a queryset list and join to this date list the count of the users of the day. The output of this query was:

<QuerySet [
{'date': datetime.date(2019, 1, 24), 'count': 12},
{'date': datetime.date(2019, 1, 9), 'count': 26},
{'date': datetime.date(2019, 1, 14), 'count': 13},
{'date': datetime.date(2019, 1, 20), 'count': 81},
{'date': datetime.date(2019, 1, 1), 'count': 7}, <<---
{'date': datetime.date(2019, 1, 7), 'count': 59},
{'date': datetime.date(2019, 1, 17), 'count': 33},
{'date': datetime.date(2019, 1, 2), 'count': 17},
{'date': datetime.date(2019, 1, 5), 'count': 59},
{'date': datetime.date(2019, 1, 12), 'count': 65},
{'date': datetime.date(2019, 1, 19), 'count': 36},
{'date': datetime.date(2019, 1, 30), 'count': 81},
{'date': datetime.date(2019, 1, 11), 'count': 35},
{'date': datetime.date(2019, 1, 18), 'count': 48},
{'date': datetime.date(2019, 1, 25), 'count': 38},
{'date': datetime.date(2019, 1, 22), 'count': 37},
{'date': datetime.date(2019, 1, 13), 'count': 2},
{'date': datetime.date(2019, 1, 3), 'count': 34},
{'date': datetime.date(2019, 1, 29), 'count': 17},
{'date': datetime.date(2019, 1, 23), 'count': 29}, '...(remaining elements truncated)...']>

Everything seemed ok until I received a support request saying that the numbers may not be correct.

On a dashboard, we have a graph with the number of new members per day on the last month, plus a big card on the side of the graph with the number of new members of the day, the bug report was that the graph and the card number were not matching. To investigate if there was, in fact, a bug I made a quick query to check the number of new users from an individual day:

# start_date = '2019-01-01'
# Result: 10

Numbers didn’t match, the bug was real.

Investigating the problem

As a developer I always fell that I’m partially a detective, catching bugs, searching their hidden motives. My first thought was: let’s check these users membership dates more closely. With the same query of before with a small tweak on the date formatting the result is:

# User.objects.filter(member_since=start_date).count() - Result: 10
['2019-01-01 01:16:36 +0000',
 '2019-01-01 01:22:41 +0000',
 '2019-01-01 01:52:39 +0000',
 '2019-01-01 10:32:11 +0000',
 '2019-01-01 13:31:50 +0000',
 '2019-01-01 14:04:57 +0000',
 '2019-01-01 17:07:00 +0000',
 '2019-01-01 18:24:38 +0000',
 '2019-01-01 19:42:22 +0000',
 '2019-01-01 20:54:14 +0000']

The results are the same as the previous individual query, showing a total of 10 users per day. However, the interesting thing here is the date format, more specifically the timezone part: +0000, this minor part indicates the timezone where the date is saved. We can see that the dates are saved in UTC, we can also see that there are 3 dates at the very beginning of the day.

Back to the old query, the filter by the field seems to be working properly, the problem may be happening on the part of annotating a new date without the hour and timezone information.

# same query as above

The TruncDate class extends from the Trunc class which truncates a date up to a significant component, in this case, the date component. Given the datetime 2019–01–01 01:16:36 +00:00, the built-in kinds return:

  • day: 2019–01–01 00:00:00+00:00

But there’s a catch if a different timezone like America/Recife is active in Django, then the datetime is converted to the new timezone before the value is truncated, resulting in:

  • day: 2018–12–31 22:16:36 -03:00
# Datetime conversion
local_tz = pytz.timezone("America/Recife")
local_dt = date.replace(tzinfo=pytz.utc).astimezone(local_tz)
new_date = local_tz.normalize(local_dt)
print(new_date.strftime('%Y-%m-%d %H:%M:%S %z'))
# Result: 2018-12-31 22:16:36 -0300

And we found the bug.

Fixing the bug

Before the system would make two queries to get basically the same information, one solution would be simply to use one query to maintain consistency. Other solution would make all operations and after that made the timezone conversion from UTC to the local timezone. The solution I made was to annotate the date with the correct timezone on the very beginning.

# start_date = '2019-01-01'
# end_date = '2019-02-01'

Not the best solution, but was the less disruptive, at the moment, for the system architecture.

Did I make a mistake? Please consider sending a pull request.