diff --git a/pkgs/top-level/make-tarball.nix b/pkgs/top-level/make-tarball.nix
index 288971403f1..7bb18e765f1 100644
--- a/pkgs/top-level/make-tarball.nix
+++ b/pkgs/top-level/make-tarball.nix
@@ -17,7 +17,10 @@ releaseTools.sourceTarball {
 
   inherit officialRelease;
   version = pkgs.lib.fileContents ../../.version;
-  versionSuffix = "pre${toString nixpkgs.revCount}.${nixpkgs.shortRev}";
+  versionSuffix = "pre${
+    if nixpkgs ? lastModified
+    then builtins.substring 0 8 nixpkgs.lastModified
+    else toString nixpkgs.revCount}.${nixpkgs.shortRev}";
 
   buildInputs = [ nix.out jq lib-tests ];