The source for the cpython build is the release tarball (https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/interpreters/python/cpython/default.nix#L

The source for the cpython build is the release tarball (https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/interpreters/python/cpython/default.nix#L155 https://github.com/NixOS/nixpkgs/blob/master/pkgs/developmen...). In that case, NixOS sets SOURCE_DATE_EPOCH (which I suspect will be picked up by the python build) to the latest timestamp found in that archive (https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/setup-hooks/set-source-date-epoch-to-latest.sh https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-supp...)