I started investigating the handling of timezones and the TZ environment variable for #712.

Turns out documentation on this matter is a sad joke and I had to read glibc (tzset.c and tzfile.c) and musl source code. Here’s what I discovered:

• An empty TZ indicates a UTC timezone
• When TZ starts with :, the character is removed from the start of the string
• When TZ is an absolute path (starts with /), it uses the file directly
• Otherwise it’s a relative path.
  • glibc searches at the directory described by TZDIR, or /usr/share/zoneinfo if it doesn’t exist
  • musl searches in /usr/share/zoneinfo/, then /share/zoneinfo/, then /etc/zoneinfo/.

Note 1: TZDIR is used by NixOS (it’s not documented anywhere but there are uses in their codebase…).
Note 2: At least one Linux distribution is using /etc/zoneinfo, apparently without TZDIR because it uses musl.