diff --git a/nix-conf/home-manager/home.nix b/nix-conf/home-manager/home.nix
index a029708..bc42a51 100644
--- a/nix-conf/home-manager/home.nix
+++ b/nix-conf/home-manager/home.nix
@@ -91,4 +91,7 @@ in
     # https://github.com/nix-community/home-manager/pull/3287
     # extraConfig = builtins.readFile "${proj_root}/neovim/init.lua";
   };
+  # not exist in home-manager
+  # have to do it at system level
+  # services.ntp.enable = true; # automatic time
 }
diff --git a/nix-conf/system/configuration.nix b/nix-conf/system/configuration.nix
index fa613d0..1cd678d 100755
--- a/nix-conf/system/configuration.nix
+++ b/nix-conf/system/configuration.nix
@@ -61,6 +61,7 @@ with lib;
   # inherit services;
   services = lib.recursiveUpdate _services {
     tailscale.enable = true;
+    ntp.enable = true;
   };
   # create a oneshot job to authenticate to Tailscale
   systemd.services.tailscale-autoconnect = {
diff --git a/scripts/hm-switch.sh b/scripts/hm-switch.sh
index c18fca9..6ff6f5b 100755
--- a/scripts/hm-switch.sh
+++ b/scripts/hm-switch.sh
@@ -12,7 +12,7 @@ HOME_MANAGER_DIR="${SCRIPT_DIR}/../nix-conf/home-manager"
 if [ $(home-manager >/dev/null 2>&1) ]; then
     nix-shell -p home-manager --run "home-manager switch --flake $HOME_MANAGER_DIR"
 else
-    home-manager switch --flake "$HOME_MANAGER_DIR"
+    home-manager switch -b backup --flake "$HOME_MANAGER_DIR"
 fi