Date objects allow you to represent dates and times in a way that can be used for date calculations and conversions. As absolute points in time, date objects are meaningful across locales, timezones, and calendars.

Date Fundamentals

Cocoa represents dates and times as NSDate objects. NSDate is one of the fundamental Cocoa value objects. A date object represents an invariant point in time. Because a date is a point in time, it implies clock time as well as a day, so there is no way to define a date object to represent a day without a time.

To understand how Cocoa handles dates, you must consider NSCalendar and NSDateComponents objects as well. In a nontechnical context, a point in time is usually represented by a combination of a clock time and a day on a particular calendar (such as the Gregorian or Hebrew calendar). Supporting different calendars is important for localization. In Cocoa, you use a particular calendar to decompose a date object into its date components such as year, month, day, hour, and minute. Conversely, you can use a calendar to create a date object from date components. Calendar and date component objects are described in more detail in Calendars, Date Components, and Calendar Units.

NSDate provides methods for creating dates, comparing dates, and computing intervals. Date objects are immutable. The standard unit of time for date objects is floating point value typed as NSTimeInterval and is expressed in seconds. This type makes possible a wide and fine-grained range of date and time values, giving precision within milliseconds for dates 10,000 years apart.

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. The sole primitive method of NSDate, timeIntervalSinceReferenceDate provides the basis for all the other methods in the NSDate interface. NSDate converts all date and time representations to and from NSTimeInterval values that are relative to the absolute reference date.

Cocoa implements time according to the Network Time Protocol (NTP) standard, which is based on Coordinated Universal Time.

Creating Date Objects

If you want a date that represents the current time, you allocate an NSDate object and initialize it with init:

NSDate *now = [[NSDate alloc] init];

or use the NSDate class method date to create the date object. If you want some time other than the current time, you can use one of NSDate’s initWithTimeInterval... or dateWithTimeInterval... methods; typically, however, you use a more sophisticated approach employing a calendar and date components as described in Calendar Basics.

The initWithTimeInterval... methods initialize date objects relative to a particular time, which the method name describes. You specify (in seconds) how much more recent or how much more in the past you want your date object to be. To specify a date that occurs earlier than the method’s reference date, use a negative number of seconds.

Listing 1 defines two date objects. The tomorrow object is exactly 24 hours from the current date and time, and yesterday is exactly 24 hours earlier than the current date and time.

Listing 1  Creating dates with time intervals

NSTimeInterval secondsPerDay = 24 * 60 * 60;
NSDate *tomorrow = [[NSDate alloc] initWithTimeIntervalSinceNow:secondsPerDay];
NSDate *yesterday = [[NSDate alloc] initWithTimeIntervalSinceNow:-secondsPerDay];

Listing 2 shows how to get new date objects with date-and-time values adjusted from existing date objects using dateByAddingTimeInterval:.

Listing 2  Creating dates by adding a time interval

NSTimeInterval secondsPerDay = 24 * 60 * 60;
NSDate *today = [[NSDate alloc] init];
NSDate *tomorrow, *yesterday;
tomorrow = [today dateByAddingTimeInterval:secondsPerDay];
yesterday = [today dateByAddingTimeInterval:-secondsPerDay];

Basic Date Calculations

To compare dates, you can use the isEqualToDate:, compare:, laterDate:, and earlierDate: methods. These methods perform exact comparisons, which means they detect sub-second differences between dates. You may want to compare dates with a less fine granularity. For example, you may want to consider two dates equal if they are within a minute of each other. If this is the case, use timeIntervalSinceDate: to compare the two dates. The following code fragment shows how to use timeIntervalSinceDate: to see if two dates are within one minute (60 seconds) of each other.

if (fabs([date2 timeIntervalSinceDate:date1]) < 60) {
    // …

To obtain the difference between a date object and another point in time, send a timeIntervalSince... message to the date object. For example, timeIntervalSinceNow gives you the time, in seconds, between the current time and the receiving date object.

To get the component elements of a date, such as the day of the week, use an NSDateComponents object in conjunction with an NSCalendar object. This technique is described in Calendar Basics.