Add Override Timezone That All Tests Run In as a Python TIL

README.md

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 - [Make Secure Temp File For Atomic Write](python/make-secure-temp-file-for-atomic-write.md)
 - [Override The Boolean Context Of A Class](python/override-the-boolean-context-of-a-class.md)
+- [Override Timezone That All Tests Run In](python/override-timezone-that-all-tests-run-in.md)
 - [Parse Relative Time To datetime Object](python/parse-relative-time-to-datetime-object.md)
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 - [Reclassify Certain Packages As Dev Dependencies](python/reclassify-certain-packages-as-dev-dependencies.md)

python/override-timezone-that-all-tests-run-in.md

@@ -0,0 +1,61 @@
+# Override Timezone That All Tests Run In
+
+I was building out the initial functionality of a Python project in my local dev
+env. I was writing tests along the way to guard against regressions. Many of the
+tests assert things about time, durations, and formatted timestamps. All was
+well.
+
+Then I was getting the project setup on GitHub with a CI workflow to run the
+test suite. And a ton of tests were failing. It was all the tests dealing with
+time. They were all off by 5 hours.
+
+I recognized that time difference immediately. That's the difference between my
+time here in Chicago and UTC. When I run the tests locally, they are all
+formatted the timezone setting on my machine -- `America/Chicago`. Whereas when
+I run them in this CI environment, they were suddenly being formatted to `UTC`.
+
+What I needed to do was explicitly set the timezone that all the tests would
+always run in, regardless of environment.
+
+I added the following to my `conftest.py` file:
+
+```python
+import os
+import time
+
+
+# Set the timezone that all tests run in
+os.environ["TZ"] = "America/Chicago"
+time.tzset()
+```
+
+First, I set the timezone (`TZ`) in `os.environ` to `America/Chicago`. That's a
+copy of the executing environment that Python makes when it starts up.
+
+Second, because `TZ` may be tied to some cached info under the hood, I have to
+also run `time.tzset()`.
+
+> Initialize, or reinitialize, the local timezone to the value stored in
+> os.environ[‘TZ’]. The TZ environment variable should be specified in standard
+> Unix timezone format as documented in the tzset man page (eg. ‘US/Eastern’,
+> ‘Europe/Amsterdam’).
+
+Now, all of my tests run reliably in `America/Chicago`.
+
+To demonstrate all of this from the Python repl, here is an example:
+
+```python
+>>> from datetime import datetime
+>>> import os
+>>> import time
+>>> os.environ["TZ"] = "America/Los_Angeles"
+>>> time.tzset()
+>>> datetime.now()
+datetime.datetime(2026, 8, 15, 14, 3, 19, 373682)
+>>> os.environ["TZ"] = "America/Chicago"
+>>> time.tzset()
+>>> datetime.now()
+datetime.datetime(2026, 8, 15, 16, 3, 33, 322803)
+```
+
+Notice the two hour difference between those two.