The problem is that can be difficult to portably determine. One wishes POSIX had an API “give me IANA time zone name for current process” which would do the needful to work it out (read TZ environment variable, readlink /etc/localtime, whatever else might be necessary)… but no, you are left to do those steps yourself. And it works reasonably well if the TZ environment variable is set, but it most commonly isn’t; readlink of /etc/localtime works on macOS and some Linux distros… but others make /etc/localtime a regular file not a symlink, which makes it all a lot harder

And that’s POSIX. Then there’s Windows which is possibly the last platform to still use its own timezone database instead of IANA’s. Now, Unicode CLDR maintains a Windows-to-IANA mapping table… but you have to ship both that table, and maybe the IANA timezone DB too, with your app, and keep them updated

I really wish Microsoft would ship the IANA database with Windows, and the IANA-Windows mapping table too, and provide APIs to query them, and keep them updated with Windows update. The core Windows OS and existing Windows apps can keep on using the legacy Windows TZ database for backward compatibility, whereas portable apps could use IANA instead if they wish