nix/nixos/common/modules/nixos.nix

{ ... }: { 
  nix = {
    settings = {
      auto-optimise-store = true;
      experimental-features = [ "nix-command"  "flakes" ];
    };
    optimise = {
      automatic = true;
      dates = [ "daily" ];
    };
    gc = {
      automatic = true;
      dates = "daily";
      options = "--delete-older-than 30d";
    };
  };
}