It's trivial to write non-reproducible Nix derivations by placing timestamps, randomly generated data, or similar into the build output, directly or indirectly. Nix is a powerful tool for working towards reproducibility, since it can specify the software side of a build environment more exhaustively than most other package managers out there, and has a language (derivations) that enables repeating builds easily as well as comparing them, but does not guarantee reproducibility on its own.
A lot of the reproducibility of nixpkgs is attributable to reproducible build efforts elsewhere, including conventions like SOURCE_DATE_EPOCH. Nix itself does not in fact "freeze the clock" for builds, but many tools that use timestamps as part of a build will respect SOURCE_DATE_EPOCH instead of using the system clock. Many of these valuable developments in this direction are coordinated in the cross-distro https://reproducible-builds.org group.