NSDate

[1] NSDate - Foundation | Apple Developer Documentation (2017-04-25 03:26:05 +09:00) <https://developer.apple.com/reference/foundation/nsdate>

Date objects are immutable, representing an invariant time interval relative to an absolute reference date (00:00:00 UTC on 1 January 2001).

[2] Dates (2017-05-11 02:12:53 +09:00) <https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/DatesAndTimes/Articles/dtDates.html>

NSDate computes time as seconds relative to an absolute reference time: the first instant of January 1, 2001, Greenwich Mean Time (GMT). Dates before then are stored as negative numbers; dates after then are stored as positive numbers.