NSDate objects represent a single point in time. NSDate is a class cluster; its single public superclass, NSDate, declares the programmatic interface for specific and relative time values. The objects you create using NSDate are referred to as date objects. They are immutable objects. Because of the nature of class clusters, objects returned by the NSDate class are instances not of that abstract class but of one of its private subclasses.
NSDate objects represent a single point in time. NSDate is a class cluster; its single public superclass, NSDate, declares the programmatic interface for specific and relative time values. The objects you create using NSDate are referred to as date objects. They are immutable objects. Because of the nature of class clusters, objects returned by the NSDate class are instances not of that abstract class but of one of its private subclasses. Although a date object’s class is private, its interface is public, as declared by the abstract superclass NSDate. Generally, you instantiate a suitable date object by invoking one of the date... class methods.
NSDate is an abstract class that provides behavior for creating dates, comparing dates, representing dates, computing intervals, and similar functionality. NSDate presents a programmatic interface through which suitable date objects are requested and returned. Date objects returned from NSDate are lightweight and immutable since they represent an invariant point in time. This class is designed to provide the foundation for arbitrary calendrical representations.
The sole primitive method of NSDate, timeIntervalSinceReferenceDate, provides the basis for all the other methods in the NSDate interface. This method returns a time value relative to an absolute reference date—the first instant of 1 January 2001, GMT.
To parse strings containing dates and to generate string representations of a date, you should use an instance of NSDateFormatter using the methods dateFromString: and stringFromDate: respectively—see Date Formatters for more details.
NSDate models the change from the Julian to the Gregorian calendar in October 1582, and calendrical calculations performed in conjunction with NSCalendar take this transition into account. Note, however, that some locales adopted the Gregorian calendar at other times; for example, Great Britain didn't switch over until September 1752.
NSDate is “toll-free bridged” with its Cocoa Foundation counterpart, CFDateRef. See Toll-Free Bridging for more information on toll-free bridging.
Subclassing Notes
The major reason for subclassing NSDate is to create a class with convenience methods for working with a particular calendrical system. But you could also require a custom NSDate class for other reasons, such as to get a date and time value that provides a finer temporal granularity. Methods to Override
If you want to subclass NSDate to obtain behavior different than that provided by the private or public subclasses, you must do these things:
Declare a suitable instance variable to hold the date and time value (relative to an absolute reference date).
Override the timeIntervalSinceReferenceDate instance method to provide the correct date and time value based on your instance variable.
Override initWithTimeIntervalSinceReferenceDate:, one of the designated initializer methods.
If you are creating a subclass that represents a calendrical system, you must also define methods that partition past and future periods into the units of this calendar.
Because the NSDate class adopts the NSCopying and NSCoding protocols, your subclass must also implement all of the methods in these protocols.
Special Considerations
Your subclass may use a different reference date than the absolute reference date used by NSDate (the first instance of 1 January 2001, GMT). If it does, it must still use the absolute reference date in its implementations of the methods timeIntervalSinceReferenceDate and initWithTimeIntervalSinceReferenceDate:. That is, the reference date referred to in the titles of these methods is the absolute reference date. If you do not use the absolute reference date in these methods, comparisons between NSDate objects of your subclass and NSDate objects of a private subclass will not work.
