string(TIMESTAMP) yields undefined %Z output on systems lacking timezone database

Answer to the core question

When the host system does not provide a time‑zone database (or the TZ environment variable is unset), string(TIMESTAMP) will not supply a fallback value. The command forwards the format string to the platform’s strftime implementation, which typically returns an empty string for %Z or, on some Unix‑like systems, leaves the literal %Z unchanged. CMake itself does not ship a time‑zone database or a hard‑coded default such as “UTC”.

Likely explanation

• strftime is responsible for formatting %Z. If it cannot look up the zone name, the standard says the result is implementation‑defined.
• On POSIX systems with no /usr/share/zoneinfo or a missing TZ variable, the result is usually an empty string.
• On Windows, %Z is often undefined and may produce an empty string or a UTC offset string, but there is no guarantee.
• Because CMake does not intervene, the output is whatever strftime gives.

Confirmed facts (from the research brief)

• CMake’s string(TIMESTAMP) relies on the host C runtime’s strftime.
• No internal fallback for %Z exists in CMake.
• Using %Z for logic‑critical parsing is unsafe across platforms.

Practical steps for this case

1. Create a minimal CMakeLists.txt that prints the timestamp with %Z:

   cmake_minimum_required(VERSION 3.20)
   project(TZTest)
   message(STATUS "Timestamp: ${string(TIMESTAMP "%Y-%m-%d %H:%M:%S %Z")}")
   

2. Run cmake -S . -B build on the target system.
3. Observe the output:
   • If the system has a valid TZ database and TZ set, you’ll see a zone abbreviation (e.g., “PDT”).
   • If the database is missing or TZ is unset, you’ll see an empty string or the literal %Z.
4. For reproducible builds, avoid relying on %Z. Use %z for numeric offsets or supply a fixed string in the format string.

What to do if you need a stable zone name

• Set TZ=UTC in your build environment before invoking CMake.
• Pass an explicit timezone identifier via a variable and use it in the format string, e.g., "%Y-%m-%d %H:%M:%S UTC".
• Consider using date or time utilities that accept a --utc flag, but this moves the logic outside CMake.

Missing diagnostic detail

To tailor the recommendation further, it would help to know the target platform’s C library (glibc, musl, ucrt) and whether the TZ variable is intentionally unset during the build. If you can provide that, a more precise fallback strategy can be suggested.