TZ Environment Variable: IANA Zone Name vs POSIX String Parsing

Direct Answer

No, the absence of a leading colon in TZ does not consistently trigger POSIX‑string parsing across all compliant C library implementations. glibc may fall back to IANA zone‑name lookup when the value does not match a valid POSIX TZ string, while musl treats a colon‑less value strictly as a POSIX string and only falls back to the system default if the string is invalid. When TZ is unset, both glibc and musl (and most other Unix‑like libc implementations) ignore TZ entirely and obtain the local time zone from /etc/localtime (or compiled‑in zone data).

Likely Explanation vs Confirmed Facts

Confirmed facts:

• When TZ is set with a leading colon (e.g., :America/New_York), all major libc implementations interpret the value as a path to an IANA zoneinfo file.
• When TZ is unset, the local time zone is derived from /etc/localtime (a symlink or copy of a zoneinfo file) or from compiled‑in zone data; the TZ variable is not consulted.

Likely explanation (implementation‑dependent behavior):

• glibc (tzset()) first attempts to parse the value as a POSIX TZ string; if the string does not conform to the POSIX grammar, glibc treats it as a possible IANA name and tries to load /usr/share/zoneinfo/. This fallback varies with glibc version and tzdata availability.
• musl’s tzset() strictly expects a POSIX TZ string when no colon is present; a non‑matching string is considered invalid, causing musl to ignore TZ and fall back to the system default zone (usually /etc/localtime).
• Other libc families (e.g., BSD libc, uClibc‑ng) follow either glibc‑like fallback or musl‑like strictness; consult the specific man 3 tzset for the exact rule.

Verification Steps for Your Environment

1. Identify the C library in use: run ldd --version (glibc) or musl --version; if neither, check /lib/libc.so.* or /usr/lib/libc.musl-*.so.1.
2. Test colon‑less IANA name:

   TZ=America/New_York date
   

   Record the output.
3. Test colon‑prefixed IANA name:

   TZ=:America/New_York date
   

   Record the output.
4. Compare the two outputs. If they differ, your libc applies glibc‑like fallback; if they are the same (and the colon‑less case yields UTC or an error), your libc follows musl‑like strict parsing.
5. Confirm the default zone when TZ is unset:

   unset TZ
    date
   

   Verify that /etc/localtime points to the expected zoneinfo file with readlink -f /etc/localtime.

If you are unsure which libc your system uses, please provide the output of ldd --version or the equivalent musl version command so the recommendation can be narrowed.