Use ISO 8601 as the default datetime format

app/events/sqlalchemy.py

@@ -96,9 +96,9 @@ def ressource_after_update_handler(mapper, connection, ressource):
         if not history.has_changes():
             continue
         new_value = history.added[0]
-        # DateTime attributes must be converted to a timestamp
+        # DateTime attributes must be converted to a string
         if isinstance(new_value, datetime):
-            new_value = new_value.timestamp()
+            new_value = new_value.isoformat()
         jsonpatch.append(
             {
                 'op': 'replace',