diff --git a/services/core-api/app/api/utils/models_mixins.py b/services/core-api/app/api/utils/models_mixins.py
index 412e4b945a..854db6129b 100644
--- a/services/core-api/app/api/utils/models_mixins.py
+++ b/services/core-api/app/api/utils/models_mixins.py
@@ -509,7 +509,7 @@ def dt_to_str(dt):
         if self.versions:
             hs = list(map(lambda version: {
                 'updated_by': version.update_user,
-                'updated_at': version.update_timestamp,
+                'updated_at': dt_to_str(version.update_timestamp),
                 'changeset': list(map(lambda key: json.loads(json.dumps({
                     'field_name': key,
                     'from': dt_to_str(version.changeset[key][0]),