TZ Environment Variable: IANA Zone Name vs POSIX String Parsing

Time Zone Selection Behavior

The TZ environment variable is used by Unix systems to determine the local time zone for functions like localtime(). According to POSIX standards, this variable can accept either a POSIX TZ string (e.g., EST5EDT) or a path to a zoneinfo file, typically prefixed with a colon to indicate an IANA zone name (e.g., :America/New_York).

Implementation Divergence

Different libc implementations handle the parsing of TZ differently when the colon prefix is omitted. While glibc may attempt to resolve a bare IANA name, other implementations like musl may treat a value without a leading colon strictly as a POSIX TZ string. This creates uncertainty when migrating applications between different Unix-like environments or container images.

Furthermore, the behavior of tm_isdst during Daylight Saving Time transitions remains implementation-defined when the local time is ambiguous or invalid.

• Does the absence of a leading colon in TZ consistently trigger POSIX string parsing across all compliant libc versions?
• How do different implementations resolve the zone name when TZ is unset and the system relies solely on /etc/localtime?