nixos/tzupdate: make enabled module actually be enabled (#361373)

nixos/modules/services/misc/tzupdate.nix

@@ -18,6 +18,25 @@ in
         update the timezone.
       '';
     };
+
+    package = lib.mkPackageOption pkgs "tzupdate" { };
+
+    timer.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = true;
+      description = ''
+        Enable the tzupdate timer to update the timezone automatically.
+      '';
+    };
+
+    timer.interval = lib.mkOption {
+      type = lib.types.str;
+      default = "hourly";
+      description = ''
+        The interval at which the tzupdate timer should run. See
+        {manpage}`systemd.time(7)` to understand the format.
+      '';
+    };
   };
 
   config = lib.mkIf cfg.enable {
@@ -26,15 +45,16 @@ in
     # zone, which is better than silently overriding it.
     time.timeZone = null;
 
-    # We provide a one-shot service which can be manually run. We could
-    # provide a service that runs on startup, but it's tricky to get
-    # a service to run after you have *internet* access.
+    # We provide a one-shot service that runs at startup once network
+    # interfaces are up, but we can’t ensure we actually have Internet access
+    # at that point. It can also be run manually with `systemctl start tzupdate`.
     systemd.services.tzupdate = {
       description = "tzupdate timezone update service";
+      wantedBy = [ "multi-user.target" ];
       wants = [ "network-online.target" ];
       after = [ "network-online.target" ];
       script = ''
-        timezone="$(${lib.getExe pkgs.tzupdate} --print-only)"
+        timezone="$(${lib.getExe cfg.package} --print-only)"
         if [[ -n "$timezone" ]]; then
           echo "Setting timezone to '$timezone'"
           timedatectl set-timezone "$timezone"
@@ -45,6 +65,17 @@ in
         Type = "oneshot";
       };
     };
+
+    systemd.timers.tzupdate = {
+      enable = cfg.timer.enable;
+      interval = cfg.timer.interval;
+      timerConfig = {
+        OnStartupSec = "30s";
+        OnCalendar = cfg.timer.interval;
+        Persistent = true;
+      };
+      wantedBy = [ "timers.target" ];
+    };
   };
 
   meta.maintainers = with lib.maintainers; [ doronbehar ];