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

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

]FIG]


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

> 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. 

]FIG]