The time zone for the calendar item.
- iOS 5.0+
- macOS 10.8+
- watchOS 2.0+
nil, the calendar item is a floating event. A floating event is not tied to a particular time zone. It occurs at a given time regardless of the time zone — for example, “lunch at noon.” The start and end times of a floating event should be set as if they were in the system time zone.