I just packaged it in my overlay so Nix users can try it out without installing it (works on macOS and Linux)

  nix run github:siraben/siraben-overlay#tz

or, when it gets upstreamed into Nixpkgs

  nix run nixpkgs#tz

Seems like a useful thing to have so you can visually see when multiple time zones have overlapping daylight! Neat!