The source for the cpython build is the release tarball (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-supp...)