The Leap Year bug that broke the build

I joined Hotjar in May of 2021. At some point in my work as a Backend Engineer I wrote a very standard looking test that checked whether a repository method (one that interacted with Postgres) returned the expected rows.

This method in particular would filter the rows based on a date range you passed as a paramter, something like this:

def get_foo_between_dates(start_date: datetime, end_date: datetime) -> list[Foo]:
    # Code goes here

The exact implementation isn’t relevant to this story, other than if you requested something outside of a valid date range it would raise an Exception.

Aligning with best practices and the strong testing culture at the time, I wrote both happy path and non-happy path tests. One of them went something like this:

1. Setup the test: create a Foo for every date between 1st and 5th January
2. Query for all Foos between 2nd and 4th January
3. Assert that I receive three Foos and that their dates are 2nd, 3rd and 4th January