EventInput.start.timeZone should be marked optional. Seems to be a typo, as the similar EventInput.end.timeZone is correctly marked as optional. Reference: https://developers.google.com/google-apps/calendar/v3/reference/events "For recurring events this field is required and specifies the time zone in which the recurrence is expanded. For single events this field is optional and indicates a custom time zone for the event start/end.".
Andy