The start date of the Gregorian calendar for the receiver.
- iOS 2.0+
- macOS 10.5+
- tvOS 9.0+
- watchOS 2.0+
This is used to specify the start date for the Gregorian calendar switch from the Julian calendar. Different locales switched at different times. Normally you should just accept the locale's default date for the switch.
NSCalendar for more information.