diff --git a/tile_gen/tile_index.py b/tile_gen/tile_index.py
index 2514c4b29d1fe7b4d676fbb787aff190a4a28d38..dfdfcc894587918744f369c03965522d6352ba85 100644
--- a/tile_gen/tile_index.py
+++ b/tile_gen/tile_index.py
@@ -110,7 +110,7 @@ def index(input_files, output_shapefile):
     shutil.rmtree(tmp_dir, ignore_errors=True)
 
     times_removed = current_times - new_times
-    times_added = new_times = current_times
+    times_added = new_times - current_times
     return sorted(times_removed), sorted(times_added)