nixos/localtimed: set time.timeZone to null to avoid silent overriding (#347221)

nixos/doc/manual/release-notes/rl-2411.section.md

@@ -676,6 +676,9 @@
 
 - ZFS now imports its pools in `postResumeCommands` rather than `postDeviceCommands`. If you had `postDeviceCommands` scripts that depended on ZFS pools being imported, those now need to be in `postResumeCommands`.
 
+- `services.localtimed.enable = true` will now set `time.timeZone = null`.
+  This is to avoid silently shadowing a user's explicitly defined timezone without recognition on the user's part.
+
 ## Detailed migration information {#sec-release-24.11-migration}
 
 ### `sound` options removal {#sec-release-24.11-migration-sound}

nixos/modules/services/system/localtimed.nix

@@ -16,6 +16,11 @@ in {
           Enable `localtimed`, a simple daemon for keeping the
           system timezone up-to-date based on the current location. It uses
           geoclue2 to determine the current location.
+
+          To avoid silent overriding by the service, if you have explicitly set a
+          timezone, either remove it or ensure that it is set with a lower priority
+          than the default value using `lib.mkDefault` or `lib.mkOverride`. This is
+          to make the choice deliberate. An error will be presented otherwise.
         '';
       };
       package = mkPackageOption pkgs "localtime" { };
@@ -24,6 +29,10 @@ in {
   };
 
   config = mkIf cfg.enable {
+    # This will give users an error if they have set an explicit time
+    # zone, rather than having the service silently override it.
+    time.timeZone = null;
+
     services.geoclue2.appConfig.localtimed = {
       isAllowed = true;
       isSystem = true;