diff --git a/flake.nix b/flake.nix
index d9653c8f..87f617be 100644
--- a/flake.nix
+++ b/flake.nix
@@ -61,6 +61,13 @@
             # See also `rust-toolchain.toml`
             sha256 = "sha256-AJ6LX/Q/Er9kS15bn9iflkUwcgYqRQxiOIL2ToVAXaU=";
           };
+        
+        # The timestamp of the last commit in ISO 8601 format
+        timestamp = pkgs.lib.strings.trim (builtins.readFile (
+          pkgs.runCommand "iso-timestamp" {} ''
+            date -u -d "@${toString inputs.self.lastModified}" +%Y-%m-%dT%H:%M:%SZ > $out
+          ''
+        ));
       });
     in
     inputs.flake-utils.lib.eachDefaultSystem (system:
diff --git a/nix/pkgs/oci-image/default.nix b/nix/pkgs/oci-image/default.nix
index 1224d41c..9845bbe3 100644
--- a/nix/pkgs/oci-image/default.nix
+++ b/nix/pkgs/oci-image/default.nix
@@ -4,6 +4,7 @@
 , inputs
 , lib
 , pkgs
+, timestamp
 }:
 let
   # See https://github.com/krallin/tini/pull/223
@@ -14,27 +15,11 @@ let
       })
     ];
   };
-
-  # This is a unix timestamp of the last commit
-  timestamp = inputs.self.lastModified;
-
-  # Convert the timestamp to an ISO 8601 string
-  iso = builtins.readFile (
-    pkgs.runCommand "timestamp" {
-      when = timestamp;
-      nativeBuildInputs = [ pkgs.coreutils ];
-    } ''
-      date -u -d "@$when" +%Y-%m-%dT%H:%M:%SZ > $out
-    ''
-  );
-
-  # Remove trailing newline
-  isoTrimmed = lib.strings.trim iso;
 in
 dockerTools.buildImage {
   name = default.pname;
   tag = "next";
-  created = isoTrimmed;
+  created = timestamp;
   copyToRoot = [
     dockerTools.caCertificates
   ];