If `current.start` is already a datetime object, `current.start.isoformat()` will return an isoformatted string. Make sure the time is utc.    We have this util to convert a datetime to utc:  https://github.com/home-assistant/home-assistant/blob/dev/homeassistant/util/dt.py#L63    https://docs.python.org/3/library/datetime.html