Timezones change all the time. There are almost 200 countries in the world, some so large that even their sub-states can and do change timezone rules on their own (US, Russia, India, China...). This is why Linux distributions have a dedicated package listing timezone info that is updated almost weekly, and Windows has similar arrangements.

Any solid datetime implementation better consider that time is not, in fact, immutable - even UTC can and will shift here and there.