diff --git a/pkgs/build-support/fetchgit/nix-prefetch-git b/pkgs/build-support/fetchgit/nix-prefetch-git
index 54945d203afb..f71d9ac55bc0 100755
--- a/pkgs/build-support/fetchgit/nix-prefetch-git
+++ b/pkgs/build-support/fetchgit/nix-prefetch-git
@@ -291,8 +291,8 @@ _clone_user_rev() {
     pushd "$dir" >/dev/null
     fullRev=$( (git rev-parse "$rev" 2>/dev/null || git rev-parse "refs/heads/$branchName") | tail -n1)
     humanReadableRev=$(git describe "$fullRev" 2> /dev/null || git describe --tags "$fullRev" 2> /dev/null || echo -- none --)
-    commitDate=$(git show --no-patch --pretty=%ci "$fullRev")
-    commitDateStrict8601=$(git show --no-patch --pretty=%cI "$fullRev")
+    commitDate=$(git show -1 --no-patch --pretty=%ci "$fullRev")
+    commitDateStrict8601=$(git show -1 --no-patch --pretty=%cI "$fullRev")
     popd >/dev/null
 
     # Allow doing additional processing before .git removal