The date is embedded in a shipped PDF file:

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/diffoscope-results/clhep.html

  ./usr/share/doc/clhep/GenericFunctions/genericFunctions.pdf.gz

  November·24,·2025
  vs.
  December·27,·2026

The attached patch sets FORCE_SOURCE_DATE in debian/rules to ensure that
texlive respects SOURCE_DATE_EPOCH for a reproducible date.

According to my local tests, with this patch applied clhep should
build reproducibly on tests.reproducible-builds.org!

Thanks for maintaining clhep!

live well,
  vagrant