The interval between the date object and 00:00:00 UTC on 1 January 1970.
- iOS 2.0+
- macOS 10.0+
- UIKit for Mac 13.0+Beta
- tvOS 9.0+
- watchOS 2.0+
This property’s value is negative if the date object is earlier than 00:00:00 UTC on 1 January 1970.
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