This is a hard-earned lesson, but it's true. Instead of doing def do_something_with_date(): now = datetime.now() return now - timedelta(days=2)

This is a hard-earned lesson, but it's true. Instead of doing def do_something_with_date(): now = datetime.now() return now - timedelta(days=2) you should do def do_something_with_date(now): return now - timedelta(days=2) and explicitly pass in edge-case dates into the `now` param in your unit tests. Alternatively, if you're using Python, use the freezegun library to fix the current time in tests: https://github.com/spulec/freezegun https://github.com/spulec/freezegun