diff --git a/nix/nixos-module/container/dns.nix b/nix/nixos-module/container/dns.nix
index 9ec4ada..a7f36b0 100644
--- a/nix/nixos-module/container/dns.nix
+++ b/nix/nixos-module/container/dns.nix
@@ -1,14 +1,7 @@
 { hostName, config, lib, pkgs, self, ... }:
 
 let
-  serial =
-    let
-      timestamp = toString self.lastModified;
-      datePkg = pkgs.runCommandLocal "date-${timestamp}" {} ''
-            date -d @${timestamp} +%Y%m%d%H > $out
-          '';
-    in
-      toString (import datePkg);
+  serial = builtins.substring 0 10 self.lastModifiedDate;
 
   generateZoneFile = { name, ns, records, dynamic }:
     builtins.toFile "${name}.zone" ''