diff --git a/swh/core/serializers.py b/swh/core/serializers.py
--- a/swh/core/serializers.py
+++ b/swh/core/serializers.py
@@ -178,7 +178,11 @@
         if b'__uuid__' in obj and obj[b'__uuid__']:
             return UUID(obj[b's'])
         if b'__timedelta__' in obj and obj[b'__timedelta__']:
-            return datetime.timedelta(**obj[b's'])
+            # for backward compatibility with old serialization method
+            if isinstance(obj[b's'], str):
+                return eval(obj[b's'])
+            else:
+                return datetime.timedelta(**obj[b's'])
         if b'__arrow__' in obj and obj[b'__arrow__']:
             return arrow.get(obj[b's'])
         return obj