diff --git a/nixos/modules/services/backup/btrbk.nix b/nixos/modules/services/backup/btrbk.nix
index 0c00b934405..e17761ffc3c 100644
--- a/nixos/modules/services/backup/btrbk.nix
+++ b/nixos/modules/services/backup/btrbk.nix
@@ -1,18 +1,36 @@
 { config, pkgs, lib, ... }:
 let
+  inherit (lib)
+    concatMapStringsSep
+    concatStringsSep
+    filterAttrs
+    flatten
+    isAttrs
+    isString
+    literalExpression
+    mapAttrs'
+    mapAttrsToList
+    mkIf
+    mkOption
+    optionalString
+    partition
+    typeOf
+    types
+    ;
+
   cfg = config.services.btrbk;
   sshEnabled = cfg.sshAccess != [ ];
   serviceEnabled = cfg.instances != { };
   attr2Lines = attr:
     let
-      pairs = lib.attrsets.mapAttrsToList (name: value: { inherit name value; }) attr;
+      pairs = mapAttrsToList (name: value: { inherit name value; }) attr;
       isSubsection = value:
-        if builtins.isAttrs value then true
-        else if builtins.isString value then false
-        else throw "invalid type in btrbk config ${builtins.typeOf value}";
-      sortedPairs = lib.lists.partition (x: isSubsection x.value) pairs;
+        if isAttrs value then true
+        else if isString value then false
+        else throw "invalid type in btrbk config ${typeOf value}";
+      sortedPairs = partition (x: isSubsection x.value) pairs;
     in
-    lib.flatten (
+    flatten (
       # non subsections go first
       (
         map (pair: [ "${pair.name} ${pair.value}" ]) sortedPairs.wrong
@@ -22,7 +40,7 @@ let
         map
           (
             pair:
-            lib.mapAttrsToList
+            mapAttrsToList
               (
                 childname: value:
                   [ "${pair.name} ${childname}" ] ++ (map (x: " " + x) (attr2Lines value))
@@ -34,7 +52,7 @@ let
     )
   ;
   addDefaults = settings: { backend = "btrfs-progs-sudo"; } // settings;
-  mkConfigFile = settings: lib.concatStringsSep "\n" (attr2Lines (addDefaults settings));
+  mkConfigFile = settings: concatStringsSep "\n" (attr2Lines (addDefaults settings));
   mkTestedConfigFile = name: settings:
     let
       configFile = pkgs.writeText "btrbk-${name}.conf" (mkConfigFile settings);
@@ -51,37 +69,42 @@ let
     '';
 in
 {
+  meta.maintainers = with lib.maintainers; [ oxalica ];
+
   options = {
     services.btrbk = {
-      extraPackages = lib.mkOption {
+      extraPackages = mkOption {
         description = "Extra packages for btrbk, like compression utilities for <literal>stream_compress</literal>";
-        type = lib.types.listOf lib.types.package;
+        type = types.listOf types.package;
         default = [ ];
-        example = lib.literalExpression "[ pkgs.xz ]";
+        example = literalExpression "[ pkgs.xz ]";
       };
-      niceness = lib.mkOption {
+      niceness = mkOption {
         description = "Niceness for local instances of btrbk. Also applies to remote ones connecting via ssh when positive.";
-        type = lib.types.ints.between (-20) 19;
+        type = types.ints.between (-20) 19;
         default = 10;
       };
-      ioSchedulingClass = lib.mkOption {
+      ioSchedulingClass = mkOption {
         description = "IO scheduling class for btrbk (see ionice(1) for a quick description). Applies to local instances, and remote ones connecting by ssh if set to idle.";
-        type = lib.types.enum [ "idle" "best-effort" "realtime" ];
+        type = types.enum [ "idle" "best-effort" "realtime" ];
         default = "best-effort";
       };
-      instances = lib.mkOption {
+      instances = mkOption {
         description = "Set of btrbk instances. The instance named <literal>btrbk</literal> is the default one.";
-        type = with lib.types;
+        type = with types;
           attrsOf (
             submodule {
               options = {
-                onCalendar = lib.mkOption {
-                  type = lib.types.str;
+                onCalendar = mkOption {
+                  type = types.nullOr types.str;
                   default = "daily";
-                  description = "How often this btrbk instance is started. See systemd.time(7) for more information about the format.";
+                  description = ''
+                    How often this btrbk instance is started. See systemd.time(7) for more information about the format.
+                    Setting it to null disables the timer, thus this instance can only be started manually.
+                  '';
                 };
-                settings = lib.mkOption {
-                  type = let t = lib.types.attrsOf (lib.types.either lib.types.str (t // { description = "instances of this type recursively"; })); in t;
+                settings = mkOption {
+                  type = let t = types.attrsOf (types.either types.str (t // { description = "instances of this type recursively"; })); in t;
                   default = { };
                   example = {
                     snapshot_preserve_min = "2d";
@@ -103,16 +126,16 @@ in
           );
         default = { };
       };
-      sshAccess = lib.mkOption {
+      sshAccess = mkOption {
         description = "SSH keys that should be able to make or push snapshots on this system remotely with btrbk";
-        type = with lib.types; listOf (
+        type = with types; listOf (
           submodule {
             options = {
-              key = lib.mkOption {
+              key = mkOption {
                 type = str;
                 description = "SSH public key allowed to login as user <literal>btrbk</literal> to run remote backups.";
               };
-              roles = lib.mkOption {
+              roles = mkOption {
                 type = listOf (enum [ "info" "source" "target" "delete" "snapshot" "send" "receive" ]);
                 example = [ "source" "info" "send" ];
                 description = "What actions can be performed with this SSH key. See ssh_filter_btrbk(1) for details";
@@ -125,7 +148,7 @@ in
     };
 
   };
-  config = lib.mkIf (sshEnabled || serviceEnabled) {
+  config = mkIf (sshEnabled || serviceEnabled) {
     environment.systemPackages = [ pkgs.btrbk ] ++ cfg.extraPackages;
     security.sudo.extraRules = [
       {
@@ -152,14 +175,14 @@ in
         (
           v:
           let
-            options = lib.concatMapStringsSep " " (x: "--" + x) v.roles;
+            options = concatMapStringsSep " " (x: "--" + x) v.roles;
             ioniceClass = {
               "idle" = 3;
               "best-effort" = 2;
               "realtime" = 1;
             }.${cfg.ioSchedulingClass};
           in
-          ''command="${pkgs.util-linux}/bin/ionice -t -c ${toString ioniceClass} ${lib.optionalString (cfg.niceness >= 1) "${pkgs.coreutils}/bin/nice -n ${toString cfg.niceness}"} ${pkgs.btrbk}/share/btrbk/scripts/ssh_filter_btrbk.sh --sudo ${options}" ${v.key}''
+          ''command="${pkgs.util-linux}/bin/ionice -t -c ${toString ioniceClass} ${optionalString (cfg.niceness >= 1) "${pkgs.coreutils}/bin/nice -n ${toString cfg.niceness}"} ${pkgs.btrbk}/share/btrbk/scripts/ssh_filter_btrbk.sh --sudo ${options}" ${v.key}''
         )
         cfg.sshAccess;
     };
@@ -169,7 +192,7 @@ in
       "d /var/lib/btrbk/.ssh 0700 btrbk btrbk"
       "f /var/lib/btrbk/.ssh/config 0700 btrbk btrbk - StrictHostKeyChecking=accept-new"
     ];
-    environment.etc = lib.mapAttrs'
+    environment.etc = mapAttrs'
       (
         name: instance: {
           name = "btrbk/${name}.conf";
@@ -177,7 +200,7 @@ in
         }
       )
       cfg.instances;
-    systemd.services = lib.mapAttrs'
+    systemd.services = mapAttrs'
       (
         name: _: {
           name = "btrbk-${name}";
@@ -199,7 +222,7 @@ in
       )
       cfg.instances;
 
-    systemd.timers = lib.mapAttrs'
+    systemd.timers = mapAttrs'
       (
         name: instance: {
           name = "btrbk-${name}";
@@ -214,7 +237,8 @@ in
           };
         }
       )
-      cfg.instances;
+      (filterAttrs (name: instance: instance.onCalendar != null)
+        cfg.instances);
   };
 
 }
diff --git a/nixos/tests/all-tests.nix b/nixos/tests/all-tests.nix
index e86dda9cb3d..84433806b48 100644
--- a/nixos/tests/all-tests.nix
+++ b/nixos/tests/all-tests.nix
@@ -62,6 +62,7 @@ in
   breitbandmessung = handleTest ./breitbandmessung.nix {};
   brscan5 = handleTest ./brscan5.nix {};
   btrbk = handleTest ./btrbk.nix {};
+  btrbk-no-timer = handleTest ./btrbk-no-timer.nix {};
   buildbot = handleTest ./buildbot.nix {};
   buildkite-agents = handleTest ./buildkite-agents.nix {};
   caddy = handleTest ./caddy.nix {};
diff --git a/nixos/tests/btrbk-no-timer.nix b/nixos/tests/btrbk-no-timer.nix
new file mode 100644
index 00000000000..4fcab8839c8
--- /dev/null
+++ b/nixos/tests/btrbk-no-timer.nix
@@ -0,0 +1,37 @@
+import ./make-test-python.nix ({ lib, pkgs, ... }:
+  {
+    name = "btrbk-no-timer";
+    meta.maintainers = with lib.maintainers; [ oxalica ];
+
+    nodes.machine = { ... }: {
+      environment.systemPackages = with pkgs; [ btrfs-progs ];
+      services.btrbk.instances.local = {
+        onCalendar = null;
+        settings.volume."/mnt" = {
+          snapshot_dir = "btrbk/local";
+          subvolume = "to_backup";
+        };
+      };
+    };
+
+    testScript = ''
+      start_all()
+
+      # Create btrfs partition at /mnt
+      machine.succeed("truncate --size=128M /data_fs")
+      machine.succeed("mkfs.btrfs /data_fs")
+      machine.succeed("mkdir /mnt")
+      machine.succeed("mount /data_fs /mnt")
+      machine.succeed("btrfs subvolume create /mnt/to_backup")
+      machine.succeed("mkdir -p /mnt/btrbk/local")
+
+      # The service should not have any triggering timer.
+      unit = machine.get_unit_info('btrbk-local.service')
+      assert "TriggeredBy" not in unit
+
+      # Manually starting the service should still work.
+      machine.succeed("echo foo > /mnt/to_backup/bar")
+      machine.start_job("btrbk-local.service")
+      machine.wait_until_succeeds("cat /mnt/btrbk/local/*/bar | grep foo")
+    '';
+  })