diff --git a/modules-package.nix b/modules-package.nix
index b36b809..d29908a 100644
--- a/modules-package.nix
+++ b/modules-package.nix
@@ -19,17 +19,14 @@ in stdenv.mkDerivation (self: {
 		];
 	};
 
-	phases = [ "unpackPhase" "patchPhase" "installPhase "];
+	phases = [ "unpackPhase" "patchPhase" "installPhase" ];
 
 	installPhase = lib.dedent ''
-		mkdir -vp "$out"
-		cp -rv * "$out/"
+		mkdir -p "$out"
+		cp -r * "$out/"
 
-		#mkdir -vp "$modules/share/nix/modules/dynix"
-		#cp --reflink=auto -rv * "$modules/share/nix/modules/dynix/"
-
-		mkdir -vp "$modules/share/nixos/modules/dynix"
-		cp --reflink=auto -rv * "$modules/share/nixos/modules/dynix/"
+		mkdir -p "$modules/share/nixos/modules/dynix"
+		cp --reflink=auto -r "$out/"* "$modules/share/nixos/modules/dynix/"
 	'';
 
 	passthru.mkDevShell = {
@@ -51,6 +48,8 @@ in stdenv.mkDerivation (self: {
 		] |> lib.concatStringsSep ":";
 	};
 
+	passthru.modulesPath = self.finalPackage.modules + "/share/nixos/modules";
+
 	passthru.tests = lib.fix (callPackage ./tests {
 		dynix = self.finalPackage;
 	}).packages;
diff --git a/modules/dynamicism/default.nix b/modules/dynamicism/default.nix
index dae5bd6..5e2b4e7 100644
--- a/modules/dynamicism/default.nix
+++ b/modules/dynamicism/default.nix
@@ -4,6 +4,9 @@ let
     mkOption
     showOption
   ;
+  inherit (lib.asserts)
+    checkAssertWarn
+  ;
   t = lib.types;
 
   inherit (import ./lib.nix { inherit lib; })
@@ -19,7 +22,6 @@ let
   opts = options.dynamicism;
 
   subOpts = lib.mapAttrs (_: metaAttr: metaAttr.configuration.options) options.dynamicism.for.valueMeta.attrs;
-  settingsFormat = pkgs.formats.yaml { };
 
   finalSettingsFor = { ... }@submod: recUpdateFoldl (optPath:
     lib.setAttrByPath optPath (lib.getAttrFromPath optPath config)
@@ -82,45 +84,62 @@ in
   #
   # Generic implementation.
   #
-  config.dynamicism.doChange = {
-    option,
-    value,
-    configuration ? builtins.getEnv "NIXOS_CONFIG",
-  }: let
-    loc = opts.doChange.loc ++ [ "(function argument)" "value" ];
-    option' = typeCheck loc convenientAttrPath option;
-    nixosAfter = evalNixos {
-      configuration = { config, ... }: {
-        imports = [
-          configuration
-          (lib.setAttrByPath option' (lib.mkOverride (-999) value))
-        ];
+  config.system.activationScripts."dynamicism-reset" = {
+    deps = [ "etc" "stdio" "specialfs" ];
+    text = ''
+      echo "DYNIX: removing existing systemd dropins"
+      # FIXME: do for each enabled submodule
+      if [[ -d /run/systemd/system ]]; then
+        rm -v /run/systemd/system/*/dynix-*.conf || true
+      fi
+    '';
+  };
+  config.dynamicism = {
+    doChange = {
+      option,
+      value,
+      configuration ? builtins.getEnv "NIXOS_CONFIG",
+    }: let
+      loc = opts.doChange.loc ++ [ "(function argument)" "value" ];
+      option' = typeCheck loc convenientAttrPath option;
+      nixosAfter = evalNixos {
+        configuration = { config, ... }: {
+          imports = [
+            configuration
+            (lib.setAttrByPath option' (lib.mkOverride (-999) value))
+          ];
 
-        environment.systemPackages = [
-          config.dynamicism.for.gotosocial.activate
-        ];
+          environment.systemPackages = [
+            config.dynamicism.for.gotosocial.activate
+          ];
+        };
       };
-    };
 
-    allActivations = lib.mapAttrsToList (name: submod: submod.activate) config.dynamicism.for;
-    allActivationScripts = pkgs.writeShellApplication {
-      name = "dynamicism-activate";
-      runtimeInputs = allActivations;
-      text = nixosAfter.config.dynamicism.for
-      |> lib.mapAttrsToList (name: submod: ''
-        echo "Activating dynamicism for ${name}"
-        ${lib.getExe submod.activate}
-      '')
-      |> lib.concatStringsSep "\n";
-    };
-  in allActivationScripts;
+      allActivations = config.dynamicism.for
+      |> lib.filterAttrs (name: submod: submod.enable)
+      |> lib.mapAttrsToList (name: submod: submod.activate);
+      allActivationScripts = pkgs.writeShellApplication {
+        name = "dynamicism-activate";
+        runtimeInputs = allActivations;
+        text = nixosAfter.config.dynamicism.for
+        |> lib.filterAttrs (name: submod: submod.enable)
+        |> lib.mapAttrsToList (name: submod: ''
+          echo "Activating dynamicism for ${name}"
+          ${lib.getExe submod.activate}
+        '')
+        |> lib.concatStringsSep "\n";
+      };
+    in allActivationScripts;
 
-  config.dynamicism.finalSettings = lib.asserts.checkAssertWarn ourAssertions [ ] (
-    recUpdateFoldlAttrs (name: { ... }@submod: finalSettingsFor submod) config.dynamicism.for
-  );
+    finalSettings = config.dynamicism.for
+    |> recUpdateFoldlAttrs (name: { ... }@submod: finalSettingsFor submod)
+    |> checkAssertWarn ourAssertions [ ];
+  };
 
   # Implementations.
   imports = [
     ./gotosocial.nix
+    ./harmonia.nix
+    #./tzupdate.nix
   ];
 }
diff --git a/modules/dynamicism/gotosocial.nix b/modules/dynamicism/gotosocial.nix
index d52e68c..cbcff2b 100644
--- a/modules/dynamicism/gotosocial.nix
+++ b/modules/dynamicism/gotosocial.nix
@@ -3,17 +3,20 @@ let
   cfg = config.dynamicism.for.gotosocial;
 
   settingsFormat = pkgs.formats.yaml { };
+  configFile = settingsFormat.generate "gotosocial-override.yml" config.services.gotosocial.settings;
 in
 {
   dynamicism.for.gotosocial = {
     source-options = [ "services.gotosocial.settings" ];
 
-    configFile = settingsFormat.generate "gotosocial-overrde.yml" config.services.gotosocial.settings;
-
-    unitDropins."gotosocial.service" = pkgs.writeText "gotosocial-override.conf" ''
-      [Service]
-      ExecStart=
-      ExecStart=${lib.getExe' pkgs.gotosocial "gotosocial"} --config-path ${cfg.configFile} start
-    '';
+    unitDropins."gotosocial.service" = pkgs.writeTextFile {
+      name = "gotosocial-override.conf";
+      text = ''
+        [Service]
+        ExecStart=
+        ExecStart=${lib.getExe' pkgs.gotosocial "gotosocial"} --config-path ${configFile} start
+      '';
+      passthru = { inherit configFile; };
+    };
   };
 }
diff --git a/modules/dynamicism/harmonia.nix b/modules/dynamicism/harmonia.nix
new file mode 100644
index 0000000..e4284d0
--- /dev/null
+++ b/modules/dynamicism/harmonia.nix
@@ -0,0 +1,26 @@
+{ pkgs, lib, config, ... }:
+let
+  cfg = config.dynamicism.for.harmonia;
+  settingsFormat = pkgs.formats.toml { };
+
+  # FIXME: referring to config.dynamicism.finalSettings.* in here
+  # makes lib.checkAssertWarn in the generic module cause infinite recursion.
+  finalSettings = config.services.harmonia.settings;
+
+  configFile = settingsFormat.generate "harmonia-override.toml" finalSettings;
+in
+{
+  dynamicism.for.harmonia = {
+    source-options = [ "services.harmonia.settings" ];
+
+    unitDropins."harmonia.service" = pkgs.writeTextFile {
+      name = "harmonia-override.conf";
+      text = ''
+        [Service]
+        Environment=CONFIG_FILE=${configFile}
+      '';
+
+      passthru = { inherit configFile; };
+    };
+  };
+}
diff --git a/modules/dynamicism/submodule.nix b/modules/dynamicism/submodule.nix
index 1b9dac9..4fe269c 100644
--- a/modules/dynamicism/submodule.nix
+++ b/modules/dynamicism/submodule.nix
@@ -65,7 +65,7 @@ in
     };
 
     unitDropins = mkOption {
-      type = t.attrsOf t.pathInStore;
+      type = t.attrsOf t.package;
       internal = true;
     };
 
@@ -82,7 +82,7 @@ in
       text = let
         doEdits = config.unitDropins
           |> lib.mapAttrsToList (service: dropin: ''
-            cat "${dropin}" | systemctl edit "${service}" --runtime --stdin
+            cat "${dropin}" | systemctl edit "${service}" --runtime --drop=dynix-${dropin.name} --stdin
           '');
         doReloads = config.unitDropins
         |> lib.mapAttrsToList (service: _: ''
diff --git a/modules/dynamicism/tzupdate.nix b/modules/dynamicism/tzupdate.nix
new file mode 100644
index 0000000..73f634d
--- /dev/null
+++ b/modules/dynamicism/tzupdate.nix
@@ -0,0 +1,20 @@
+{ pkgs, lib, config, ... }: let
+  cfg = config.dynamicism.for.tzupdate;
+
+
+  # FIXME: referring to config.dynamicism.finalSettings.* in here
+  # makes lib.checkAssertWarn in the generic module cause infinite recursion.
+  #finalSettings = config.dynamicism.finalSettings.tzupdate;
+  finalSettings = config.services.tzupdate.timer;
+in
+{
+  dynamicism.for.tzupdate = {
+    source-options = [ "services.tzupdate.timer" ];
+
+    unitDropins."tzupdate.timer" = pkgs.writeText "tzupdate-timer-override.conf" ''
+      [Timer]
+      OnCalendar=
+      OnCalendar=${finalSettings.interval}
+    '';
+  };
+}
diff --git a/tests/default.nix b/tests/default.nix
index a6aea2d..40369b9 100644
--- a/tests/default.nix
+++ b/tests/default.nix
@@ -10,22 +10,40 @@
   runDynixTest = testModule: pkgs.testers.runNixOSTest {
     imports = [ testModule ];
 
-    # Note: these are arguments to the *test* modules.
-    # Not the NixOS modules for the NixOS configuration the test is testing.
-    # Wew.
-    _module.args = { inherit dynix; };
-
     # Why is this argument called "extraBaseModule**s**" but take a single module argument...
-    extraBaseModules = { name, ... }: {
-      #imports = [ dynixInjectionModule ];
-      config.environment.systemPackages = [ dynix ];
+    # Also note this is an extra base module for each node of the test,
+    # not an extra test module.
+    extraBaseModules = { name, config, options, modulesPath, ... }: {
+      imports = (import "${modulesPath}/module-list.nix") ++ [
+        ./module-allow-rebuild-in-vm.nix
+        "${modulesPath}/testing/test-instrumentation.nix"
+        (toString dynix)
+      ];
+      environment.systemPackages = [ dynix ];
+
+      systemd.services."install-dynix" = {
+        enable = true;
+        serviceConfig.Type = "oneshot";
+        serviceConfig.RemainAfterExit = true;
+        path = [ config.system.path ];
+        wantedBy = [ "multi-user.target" ];
+        after = [ "default.target" ];
+        script = ''
+          nix profile install -vv "$(realpath /run/current-system/sw/share/nixos/modules/dynix/)"
+        '';
+      };
+
+      passthru = { inherit options; };
+
       # Just making something in this strict in `name`,
       # which is only present as an argument for nodes and I don't want to
       # confuse that with the test modules.
-      config.warnings = builtins.seq name [ ];
+      warnings = builtins.seq name [ ];
     };
   };
 
 in lib.makeScope lib.callPackageWith (self: {
   gotosocial = runDynixTest ./gotosocial/test.nix;
+  harmonia = runDynixTest ./harmonia/test.nix;
+  #tzupdate = runDynixTest ./tzupdate/test.nix;
 })
diff --git a/tests/gotosocial/configuration-package.nix b/tests/gotosocial/configuration-package.nix
index e6bb010..96a89d8 100644
--- a/tests/gotosocial/configuration-package.nix
+++ b/tests/gotosocial/configuration-package.nix
@@ -1,9 +1,7 @@
 {
   runCommand,
-}: runCommand "tests-basic-configuration-dot-nix" {
+}: runCommand "tests-gotosocial-configuration-dot-nix" {
 } ''
   set -euo pipefail
-  mkdir -vp "$out/share/nixos"
-  cp -rv ${./configuration.nix} "$out/share/nixos/configuration.nix"
-  #cp -rv ${../../modules/dynamicism} "$out/share/nixos/dynamicism"
+  install -Dm a=r ${./configuration.nix} "$out/share/nixos/configuration.nix"
 ''
diff --git a/tests/gotosocial/configuration.nix b/tests/gotosocial/configuration.nix
index bc751ed..deef056 100644
--- a/tests/gotosocial/configuration.nix
+++ b/tests/gotosocial/configuration.nix
@@ -18,7 +18,7 @@ in
   system.switch.enable = true;
   documentation.enable = false;
 
-  networking.hostName = "machine";
+  networking.hostName = "gotosocial-machine";
 
   boot.loader.grub = {
     enable = true;
@@ -47,7 +47,7 @@ in
     enable = true;
     setupPostgresqlDB = true;
     settings = {
-      application-name = "gotosocial-for-${name}";
+      application-name = "gotosocial-for-machine";
       host = "${name}.local";
     };
   };
diff --git a/tests/gotosocial/test-script.py b/tests/gotosocial/test-script.py
index 03974fc..d8b2af8 100644
--- a/tests/gotosocial/test-script.py
+++ b/tests/gotosocial/test-script.py
@@ -34,7 +34,6 @@ def run_log(machine: Machine, *commands: str, timeout: int | None = 60) -> str:
 def get_config_file() -> str:
     machine.wait_for_unit("gotosocial.service")
     gotosocial_pid = int(machine.get_unit_property("gotosocial.service", "MainPID"))
-    print(f"{gotosocial_pid=}")
 
     cmdline = machine.succeed(f"cat /proc/{gotosocial_pid}/cmdline")
     cmdline_args = cmdline.split("\0")
@@ -53,8 +52,7 @@ def get_config_file() -> str:
 machine.wait_for_unit("default.target")
 assert "lix" in machine.succeed("nix --version").lower()
 machine.log("INIT")
-
-machine.succeed("nix profile install -vv $(realpath /run/current-system/sw/share/nixos/modules/dynix)")
+run_log(machine, "journalctl --no-pager -eu install-dynix.service")
 
 machine.succeed("nixos-generate-config")
 machine.succeed("mkdir -vp /etc/nixos")
diff --git a/tests/gotosocial/test.nix b/tests/gotosocial/test.nix
index 4356685..9628b93 100644
--- a/tests/gotosocial/test.nix
+++ b/tests/gotosocial/test.nix
@@ -1,7 +1,7 @@
-{ dynix, ... }:
+{ ... }:
 
 {
-  name = "nixos-test-dynamicism-main";
+  name = "nixos-test-dynamicism-gotosocial";
 
   defaults = { ... }: { };
 
@@ -9,27 +9,12 @@
     p.beartype
   ];
 
-  nodes.machine = { pkgs, config, ... }: {
+  nodes.machine = { pkgs, ... }: {
     # NOTE: Anything in this `nodes.machine = ` module will not be included
     # in the VM's NixOS configuration once it does `nixos-rebuild switch`,
     # except for `./configuration.nix` which will be copied to `/etc/nixos/`.
     # dynix will also be statefully installed to root's user profile.
-    imports = [
-      ./configuration.nix
-      (toString dynix)
-    ];
-
-    system.includeBuildDependencies = true;
-    system.switch.enable = true;
-
-    virtualisation.additionalPaths = [ config.system.build.toplevel ];
-    virtualisation = {
-      memorySize = 4096;
-      cores = 4;
-      writableStore = true;
-      mountHostNixStore = true;
-      installBootLoader = true;
-    };
+    imports = [ ./configuration.nix ];
 
     environment.systemPackages = let
       configFileTree = pkgs.callPackage ./configuration-package.nix { };
@@ -38,7 +23,5 @@
     ];
   };
 
-  # What's a little IFD between friends?
-  testScript = ./test-script.py
-  |> builtins.readFile;
+  testScript = builtins.readFile ./test-script.py;
 }
diff --git a/tests/harmonia/configuration.nix b/tests/harmonia/configuration.nix
new file mode 100644
index 0000000..7bf19bb
--- /dev/null
+++ b/tests/harmonia/configuration.nix
@@ -0,0 +1,72 @@
+{ pkgs, lib, config, modulesPath, ... }:
+let
+  moduleList = import "${modulesPath}/module-list.nix";
+
+  dynixFromSearchPath = let
+    res = builtins.tryEval <dynix>;
+  in lib.optional res.success res.value;
+in
+{
+  imports = [
+    "${modulesPath}/testing/test-instrumentation.nix"
+    ./hardware-configuration.nix
+  ] ++ lib.concatLists [
+    dynixFromSearchPath
+    moduleList
+  ];
+
+  dynamicism.for.harmonia.enable = true;
+  services.harmonia = {
+    enable = true;
+    settings = {
+      # Default.
+      workers = 4;
+      # Default.
+      max_connection_rate = 256;
+    };
+  };
+
+  system.switch.enable = true;
+  documentation.enable = false;
+
+  networking.hostName = "harmonia-machine";
+
+  boot.loader.grub = {
+    enable = true;
+    device = "/dev/vda";
+    forceInstall = true;
+  };
+
+  nix = {
+    package = pkgs.lixPackageSets.latest.lix;
+    nixPath = [
+      "nixpkgs=${pkgs.path}"
+      "/nix/var/nix/profiles/per-user/root/profile/share/nixos/modules"
+    ];
+
+    settings = {
+      experimental-features = [ "nix-command" "pipe-operator" ];
+      substituters = lib.mkForce [ ];
+      hashed-mirrors = null;
+      connect-timeout = 1;
+      # For my debugging purposes.
+      show-trace = true;
+    };
+  };
+
+  environment.pathsToLink = [ "/share" ];
+  environment.extraOutputsToInstall = [ "modules" ];
+  environment.variables = {
+    "NIXOS_CONFIG" = "/etc/nixos/configuration.nix";
+  };
+
+  environment.shellAliases = {
+    ls = "eza --long --header --group --group-directories-first --classify --binary";
+  };
+
+  environment.systemPackages = with pkgs; [
+    eza
+    fd
+    ripgrep
+  ];
+}
diff --git a/tests/harmonia/hardware-configuration.nix b/tests/harmonia/hardware-configuration.nix
new file mode 100644
index 0000000..facb35d
--- /dev/null
+++ b/tests/harmonia/hardware-configuration.nix
@@ -0,0 +1,4 @@
+{ ... }:
+{
+
+}
diff --git a/tests/harmonia/test-script.py b/tests/harmonia/test-script.py
new file mode 100644
index 0000000..a91bf78
--- /dev/null
+++ b/tests/harmonia/test-script.py
@@ -0,0 +1,125 @@
+import functools
+from pathlib import Path
+from pprint import pformat
+import shlex
+import textwrap
+import tomllib
+from typing import Any, cast, TYPE_CHECKING
+
+from beartype import beartype
+
+from test_driver.machine import Machine
+from test_driver.errors import RequestedAssertionFailed
+
+if TYPE_CHECKING:
+    global machine
+    machine = cast(Machine, ...)
+    assert machine.shell is not None
+
+ls = "eza -lah --color=always --group-directories-first"
+
+indent = functools.partial(textwrap.indent, prefix='  ')
+
+@beartype
+def run_log(machine: Machine, *commands: str, timeout: int | None = 60) -> str:
+    output = ""
+    for command in commands:
+        with machine.nested(f"must succeed: {command}"):
+            (status, out) = machine.execute(f"{command} | tee /dev/stderr", timeout=timeout)
+            if status != 0:
+                machine.log(f"output: {out}")
+                raise RequestedAssertionFailed(
+                    f"command `{command}` failed (exit code {status})",
+                )
+            output += out
+
+    return output
+
+@beartype
+def get_config_file() -> dict[str, Any]:
+    machine.wait_for_unit("harmonia.service")
+    pid = int(machine.get_unit_property("harmonia.service", "MainPID"))
+    env_lines: list[str] = machine.succeed(f"cat /proc/{pid}/environ").replace("\0", "\n").splitlines()
+    pairs: list[list[str]] = [line.split("=", maxsplit=1) for line in env_lines]
+    env = dict(pairs)
+
+    config_file = Path(env["CONFIG_FILE"])
+
+    machine.log(f"copying from VM: {config_file=}")
+    machine.copy_from_vm(config_file.as_posix())
+
+    config_file_path = machine.out_dir / config_file.name
+    with open(config_file_path, "rb") as f:
+        config_data = tomllib.load(f)
+
+    config_file_path.unlink()
+    return config_data
+
+machine.wait_for_unit("default.target")
+assert "lix" in machine.succeed("nix --version").lower()
+machine.log("INIT")
+
+# Config should have our initial values.
+config_toml = get_config_file()
+machine.log(f"config.toml BEFORE first rebuild (initial): {indent(pformat(config_toml))}")
+assert int(config_toml['workers']) == 4, f"{config_toml['workers']=} != 4"
+assert int(config_toml['max_connection_rate']) == 256, f"{config_toml['max_connection_rate']=} != 256"
+
+with machine.nested("must succeed: initial nixos-rebuild switch"):
+    machine.succeed("nixos-generate-config")
+    machine.succeed("mkdir -vp /etc/nixos")
+    # Dereference is required since that configuration.nix is probably a symlink to the store.
+    machine.succeed("cp -rv --dereference /run/current-system/sw/share/nixos/configuration.nix /etc/nixos/")
+    machine.succeed("env PAGER= nixos-rebuild switch --log-format raw-with-logs --fallback")
+
+# Config should not have changed.
+config_toml = get_config_file()
+machine.log(f"config.toml after first rebuild: {indent(pformat(config_toml))}")
+assert int(config_toml['workers']) == 4, f"{config_toml['workers']=} != 4"
+assert int(config_toml['max_connection_rate']) == 256, f"{config_toml['max_connection_rate']=} != 256"
+
+new_workers = 20
+expr = textwrap.dedent(f"""
+    let
+        nixos = import <nixpkgs/nixos> {{ }};
+    in nixos.config.dynamicism.doChange {{
+        option = "services.harmonia.settings.workers";
+        value = {new_workers};
+    }}
+""").strip()
+machine.succeed(rf"""
+    nix run --show-trace --log-format raw-with-logs --impure -E {shlex.quote(expr)}
+""".strip())
+
+# Workers, but not max connection rate, should have changed.
+config_toml = get_config_file()
+machine.log(f"config.toml after DYNAMIC ACTIVATION: {indent(pformat(config_toml))}")
+assert int(config_toml['workers']) == new_workers, f"{config_toml['workers']=} != {new_workers}"
+assert int(config_toml['max_connection_rate']) == 256, f"{config_toml['max_connection_rate']=} != 256"
+
+new_max_connection_rate = 100
+expr = textwrap.dedent(f"""
+    let
+        nixos = import <nixpkgs/nixos> {{ }};
+    in nixos.config.dynamicism.doChange {{
+        option = [ "services" "harmonia" "settings" "max_connection_rate" ];
+        value = {new_max_connection_rate};
+    }}
+""").strip()
+machine.succeed(rf"""
+    nix run --show-trace --log-format raw-with-logs --impure -E {shlex.quote(expr)}
+""".strip())
+
+# Max connection rate should have changed, but workers should have reverted.
+config_toml = get_config_file()
+machine.log(f"config_toml after SECOND dynamic activation: {indent(pformat(config_toml))}")
+assert int(config_toml['max_connection_rate']) == new_max_connection_rate, f"{config_toml['max_connection_rate']=} != {new_max_connection_rate}"
+
+# And this should set everything back.
+machine.succeed("env PAGER= nixos-rebuild switch --log-format raw-with-logs --fallback")
+machine.systemctl("restart harmonia.service")
+machine.wait_for_unit("harmonia.service")
+config_toml = get_config_file()
+machine.log(f"config_toml after NORMAL activation: {indent(pformat(config_toml))}")
+assert int(config_toml['max_connection_rate']) == 256, f'{config_toml["max_connection_rate"]=} != 256'
+assert int(config_toml['workers']) == 4, f'{config_toml["workers"]=} != 4'
diff --git a/tests/harmonia/test.nix b/tests/harmonia/test.nix
new file mode 100644
index 0000000..0aea418
--- /dev/null
+++ b/tests/harmonia/test.nix
@@ -0,0 +1,23 @@
+{ ... }:
+{
+  name = "nixos-test-dynamicism-harmonia";
+
+  defaults = { ... }: { };
+
+  extraPythonPackages = p: [ p.beartype ];
+
+  nodes.machine = { name, pkgs, ... }: {
+    imports = [ ./configuration.nix ];
+
+    environment.systemPackages = let
+      configFileTree = pkgs.runCommand "${name}-configuration-dot-nix" { } ''
+        set -euo pipefail
+        install -Dm a=r ${./configuration.nix} "$out/share/nixos/configuration.nix"
+      '';
+    in [
+      configFileTree
+    ];
+  };
+
+  testScript = builtins.readFile ./test-script.py;
+}
diff --git a/tests/module-allow-rebuild-in-vm.nix b/tests/module-allow-rebuild-in-vm.nix
new file mode 100644
index 0000000..3ec14d5
--- /dev/null
+++ b/tests/module-allow-rebuild-in-vm.nix
@@ -0,0 +1,31 @@
+{ name, config, ... }:
+
+{
+  system.includeBuildDependencies = true;
+  system.switch.enable = true;
+
+  documentation.enable = false;
+
+  virtualisation = {
+    additionalPaths = [ config.system.build.toplevel ];
+    writableStore = true;
+    mountHostNixStore = true;
+    installBootLoader = true;
+
+    # With how much memory Nix eval uses, this is essentially required.
+    memorySize = 4096;
+    cores = 4;
+
+  };
+
+  boot.loader.grub = {
+    enable = true;
+    device = "/dev/vda";
+    forceInstall = true;
+  };
+
+  # Just making something in this strict in `name`,
+  # which is only present as an argument for nodes and I don't want to
+  # confuse that with the test modules.
+  warnings = builtins.seq name [ ];
+}
diff --git a/tests/tzupdate/configuration.nix b/tests/tzupdate/configuration.nix
new file mode 100644
index 0000000..176975c
--- /dev/null
+++ b/tests/tzupdate/configuration.nix
@@ -0,0 +1,67 @@
+{ pkgs, lib, config, modulesPath, ... }:
+let
+  name = config.networking.hostName;
+  moduleList = import "${modulesPath}/module-list.nix";
+
+  dynixFromSearchPath = let
+    res = builtins.tryEval <dynix>;
+  in lib.optional res.success res.value;
+in
+{
+  imports = [
+    "${modulesPath}/testing/test-instrumentation.nix"
+    ./hardware-configuration.nix
+  ] ++ lib.concatLists [
+    moduleList
+    dynixFromSearchPath
+  ];
+
+  dynamicism.for.tzupdate.enable = true;
+  services.tzupdate = {
+    enable = true;
+  };
+
+  system.switch.enable = true;
+  documentation.enable = false;
+
+  networking.hostName = "tzupdate-machine";
+
+  boot.loader.grub = {
+    enable = true;
+    device = "/dev/vda";
+    forceInstall = true;
+  };
+
+  nix = {
+    package = pkgs.lixPackageSets.latest.lix;
+    nixPath = [
+      "nixpkgs=${pkgs.path}"
+      "/nix/var/nix/profiles/per-user/root/profile/share/nixos/modules"
+    ];
+
+    settings = {
+      experimental-features = [ "nix-command" "pipe-operator" ];
+      substituters = lib.mkForce [ ];
+      hashed-mirrors = null;
+      connect-timeout = 1;
+      # For my debugging purposes.
+      show-trace = true;
+    };
+  };
+
+  environment.pathsToLink = [ "/share" ];
+  environment.extraOutputsToInstall = [ "modules" ];
+  environment.variables = {
+    "NIXOS_CONFIG" = "/etc/nixos/configuration.nix";
+  };
+
+  environment.shellAliases = {
+    ls = "eza --long --header --group --group-directories-first --classify --binary";
+  };
+
+  environment.systemPackages = with pkgs; [
+    eza
+    fd
+    ripgrep
+  ];
+}
diff --git a/tests/tzupdate/hardware-configuration.nix b/tests/tzupdate/hardware-configuration.nix
new file mode 100644
index 0000000..facb35d
--- /dev/null
+++ b/tests/tzupdate/hardware-configuration.nix
@@ -0,0 +1,4 @@
+{ ... }:
+{
+
+}
diff --git a/tests/tzupdate/test-script.py b/tests/tzupdate/test-script.py
new file mode 100644
index 0000000..53762a0
--- /dev/null
+++ b/tests/tzupdate/test-script.py
@@ -0,0 +1,52 @@
+#from pathlib import Path
+#import shlex
+#import textwrap
+from typing import cast, TYPE_CHECKING
+
+from beartype import beartype
+
+from test_driver.machine import Machine
+from test_driver.errors import RequestedAssertionFailed
+
+if TYPE_CHECKING:
+    global machine
+    machine = cast(Machine, ...)
+    assert machine.shell is not None
+
+ls = "eza -lah --color=always --group-directories-first"
+
+@beartype
+def run_log(machine: Machine, *commands: str, timeout: int | None = 60) -> str:
+    output = ""
+    for command in commands:
+        with machine.nested(f"must succeed: {command}"):
+            (status, out) = machine.execute(f"{command} | tee /dev/stderr", timeout=timeout)
+            if status != 0:
+                machine.log(f"output: {out}")
+                raise RequestedAssertionFailed(
+                    f"command `{command}` failed (exit code {status})",
+                )
+            output += out
+
+    return output
+
+machine.wait_for_unit("default.target")
+assert "lix" in machine.succeed("nix --version").lower()
+machine.log("INIT")
+
+
+machine.succeed("nixos-generate-config")
+machine.succeed("mkdir -vp /etc/nixos")
+# Dereference is required since that configuration.nix is probably a symlink to the store.
+machine.succeed("cp -rv --dereference /run/current-system/sw/share/nixos/configuration.nix /etc/nixos/")
+
+machine.succeed("env PAGER= nixos-rebuild switch --log-format raw-with-logs --fallback")
+
+@beartype
+def get_interval() -> str:
+    prop = machine.get_unit_property("tzupdate.timer", "OnCalendar")
+    print(f"{prop=}")
+
+    return prop
+
+get_interval()
diff --git a/tests/tzupdate/test.nix b/tests/tzupdate/test.nix
new file mode 100644
index 0000000..2d42c10
--- /dev/null
+++ b/tests/tzupdate/test.nix
@@ -0,0 +1,24 @@
+{ ... }:
+
+{
+  name = "nixos-test-dynamicism-tzupdate";
+
+  extraPythonPackages = p: [
+    p.beartype
+  ];
+
+  nodes.machine = { name, pkgs, ... }: {
+    imports = [ ./configuration.nix ];
+
+    environment.systemPackages = let
+      configFileTree = pkgs.runCommand "${name}-configuration-dot-nix" { } ''
+        set -euo pipefail
+        install -Dm a=r ${./configuration.nix} "$out/share/nixos/configuration.nix"
+      '';
+    in [
+      configFileTree
+    ];
+  };
+
+  testScript = builtins.readFile ./test-script.py;
+}