string(TIMESTAMP: inconsistent handling of local vs UTC time zones

Answer the exact question first

1. Can you request a specific time zone directly from string(TIMESTAMP)? No. The command only accepts an optional UTC flag. To get another zone you must set the TZ environment variable before CMake runs or compute the value externally.

2. How does CMake handle daylight‑saving transitions? It delegates to the host’s C runtime (localtime/strftime), so DST rules come from the OS’s tzdata database. The result is deterministic only if the same tzdata version is present on every machine.

3. What about Windows? The MSVC runtime honors TZ only in a very limited form and does not use the IANA tz database. MinGW/Posix‑style builds on Windows behave more like Linux, but the set of supported zone names is narrower.

Likely explanation

Because string(TIMESTAMP) relies on the host’s local‑time machinery, any change to TZ, the system clock, or the tzdata database after configure time will not affect already‑computed timestamps. This is why reproducible builds often embed a deterministic UTC stamp or an externally supplied value instead of using string(TIMESTAMP) directly.

Confirmed facts

• The command signature is string(TIMESTAMP <var> <format> [UTC]) – no zone name or offset option.
• When UTC is omitted, the C runtime’s localtime is used, pulling the current local zone from the OS.
• On POSIX systems TZ can be set to a TZ string (e.g., America/New_York) or an offset (e.g., +0200) before invoking CMake.
• Windows’ MSVC runtime recognizes only a subset of TZ syntax and does not consult the IANA database.
• Timestamps are calculated once at configure time; subsequent changes to TZ or the clock have no effect until CMake is rerun.

Steps for reproducible timestamps in this case

1. Prefer UTC for deterministic output. Add the UTC flag to the string(TIMESTAMP) call.

string(TIMESTAMP BUILD_TIME "%Y-%m-%d %H:%M:%S UTC" UTC)

2. When a local zone is required, set TZ before configuring. For example, in a shell script:

export TZ=America/Los_Angeles
cmake -S . -B build

3. For reproducible builds that need a specific offset, compute the value externally. Use a small script that calls the operating system’s date or a language runtime that supports IANA zones, then pass the result to CMake via a cache variable or an environment variable.
4. Avoid relying on the system clock. If you want to embed a build date that does not change with the host clock, consider using SOURCE_DATE_EPOCH (if available) or a CI‑provided timestamp.

Diagnostic question

To tailor the recommendation to your environment, could you tell me which CMake generator (e.g., Unix Makefiles, Visual Studio 17 2022, MinGW Makefiles) and operating system you are using? The behavior of TZ and the available zone names differ across these contexts.