[client] fix

Author: SarahBocognano

pycti/utils/opencti_stix2.py

@@ -1507,7 +1507,16 @@ def generate_export(self, entity: Dict, no_custom_attributes: bool = False) -> D
             and "valid_until" in entity
             and entity["valid_from"] == entity["valid_until"]
         ):
-            entity["valid_until"] = entity["valid_from"] + datetime.timedelta(seconds=1)
+            valid_until_converted_datetime = datetime.strptime(
+                entity["valid_until"], "%Y-%m-%dT%H:%M:%S.%fZ"
+            )
+            new_valid_until = valid_until_converted_datetime + datetime.timedelta(
+                seconds=1
+            )
+            valid_until_converted_string = new_valid_until.strftime(
+                "%Y-%m-%dT%H:%M:%S.%fZ"
+            )
+            entity["valid_until"] = valid_until_converted_string
 
         # Flatten
         if "tasks" in entity: