Add blog about pytz (#924)

blog/2021-05-07-switch-pytz-to-python-dateutil.md

@@ -0,0 +1,29 @@
+---
+author: Paulus Schoutsen
+authorURL: https://twitter.com/balloob
+authorImageURL: /img/profile/paulus.jpg
+authorTwitter: balloob
+title: Replacing pytz with python-dateutil
+---
+
+Three years ago Paul Ganssle wrote [a comparison](https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html) about time zone handling between `pytz` and `python-dateutil`. In this article he shows how it's easy to use `pytz` in an incorrect way that is hard to spot because it's almost correct:
+
+```python
+import pytz
+from datetime import datetime, timedelta
+
+NYC = pytz.timezone('America/New_York')
+dt = datetime(2018, 2, 14, 12, tzinfo=NYC)
+print(dt)
+# 2018-02-14 12:00:00-04:56
+```
+
+_[(link to part of the article explaining why it's -4:56)](https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html#pytz-s-time-zone-model)_
+
+In Home Assistant 2021.6 we're going to switch to `python-dateutil`. You will need to upgrade your custom integration if it relies on the unofficial interface `my_time_zone.localize(my_dt)`. Use Python's official method `my_dt.astimezone(my_time_zone)` instead.
+
+The property `hass.config.time_zone` will also change to a string instead of a time zone object.
+
+Thanks to [@bdraco] for helping revive this effort and push this change [past the finish line](https://github.com/home-assistant/core/pull/49643). We actually found a couple of bugs during the migration! Also thanks to Paul Ganssle for maintaining `python-dateutil` and the excellent write up.
+
+[@bdraco]: https://github.com/bdraco