This is the nichest of niche edgecases so don't feel bad about being unaware of it. It's extremely arcane knowledge that developers shouldn't generally bother learning... but it's an excellent example of my initial statement. The maintainers of tz_database know this shit and plenty of other obscure bullshit they know it and build it into their library of functions so that we don't need to worry about it. And that is an excellent reason to not reinvent the wheel.