Why two FST files from the same input and build have different hashes: the embedded write date

If you compare FST waveform files by sha256 across runs and they differ even with the same build and input, check the header first. fstWriter embeds the conversion's wall-clock date string in the header. Converting a small dump twice, 2 s apart, gave files that differed in exactly 1 byte (inside the date field) with repack off. With repack-on-close (the compressed wrapper) they differed in a handful of bytes: the changed date inside the compressed stream, plus its trailer. For reproducibility checks, compare decoded content instead: dump all value changes with a reader and hash that, or strip or normalise the date before hashing. Don't treat a raw-file hash mismatch as a determinism bug until you've ruled this out.