Cocoa. Date and Time

This small program detects the current date:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

     NSDate* today = [[NSDate alloc] init];
     NSLog(@"today is: %@", today);
    
    [today release];        
    [pool drain];
    return 0;
}

In the console you'll see:
run
[Switching to process 12401 local thread 0x3f03]
Running…
2010-03-06 17:05:45.217 DayOfToday[12401:a0f] today is: 2010-03-06 17:05:45 +0200

NSDate object (today) was created and initialized with the current date and time:

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

And the next line prints out this information:

NSLog(@"today is: %@", today);

Same result will be here:

NSDate *today = [[NSDate alloc] init];
NSString *text = [today description];
NSLog(text);

Method description gives a string representing the date.
The following program shows to calculate other related dates:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSDate *today = [[NSDate alloc] init];
    NSLog(@"Today is:     %@", today);
    
    //Number os seconds in a day:
    NSTimeInterval secondsPerDay = 24 * 60 * 60;

    //Calculates the date of tomorrow:
    NSDate *tomorrow = [today addTimeInterval: secondsPerDay];
    NSLog(@"Tomorrow is:  %@", tomorrow);
    
    //Calculates the date of yesterday:
    NSDate *yesterday = [today addTimeInterval: -secondsPerDay];
    NSLog(@"Yesterday is: %@", yesterday);
    
    NSLog(@"Verify today: %@", today);
    
    [today release];        
    [pool drain];
    return 0;
}

There is another very simple way to create the date objects:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSString *dateString = @"2010-03-12 14:07:00 +0200";    
    NSDate *today = [[NSDate alloc] initWithString: dateString];
    NSLog(@"Today is:     %@", today);
    
    [today release];        
    [pool drain];
    return 0;
}

Of course, this line:

NSString *dateString = @"2010-03-12 14:07:00 +0200";

can be shorter. For example, like that:

NSDate *today = [[NSDate alloc] initWithString: @"2010-03-12 14:07:00 +0200"];

Or simply:

NSDate *today = [NSDate dateWithString: @"2010-03-12 14:07:00 +0200"];

A special class NSDateFormatter allows to format the output:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSDate *today = [[NSDate alloc] init];
    
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateStyle: NSDateFormatterMediumStyle];

    NSLog(@"%@", [dateFormatter stringFromDate: today]);

    [dateFormatter release];
    [today release];
    [pool drain];
    return 0;
}

The output will be:

run

[Switching to process 13097 local thread 0x4003]

Running…

2010-03-06 18:19:40.752 DayOfToday[13097:a0f] Mar 6, 2010

Method compare allows to compare two NSDate objects:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSDate *d1 = [NSDate dateWithString: @"2010-03-06 01:01:01 +0200"];
    NSDate *d2 = [NSDate dateWithString: @"2010-03-12 02:02:02 +0200"];

    NSComparisonResult result = [d1 compare: d2];
    
    switch (result) {
        case NSOrderedAscending:
            NSLog(@"Ascending (d2 is a later date than d1)");
            break;
            
        case NSOrderedDescending:
            NSLog(@"Descending (d1 is a later date than d2)");
            break;
            
        case NSOrderedSame:
            NSLog(@"Same (d1 and d2 is equal)");
            break;
    }
    [pool drain];
    return 0;
}

And here is a program sorting an array of dates:

#import <Foundation/Foundation.h>

NSComparisonResult dateSort(NSDate *d1, NSDate *d2, void *context) {
     return [d1 compare:d2];
}

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSTimeInterval secondsPerDay = 24 * 60 * 60;
    NSDate *today = [[NSDate alloc] init];
    NSDate *tomorrow = [today addTimeInterval:secondsPerDay];
    NSDate *afterTomorrow = [today addTimeInterval: 2 * secondsPerDay];
    
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateStyle:NSDateFormatterMediumStyle];
    
    NSArray  *array = [NSArray arrayWithObjects: tomorrow, today, afterTomorrow, nil];
    [array sortedArrayUsingFunction: dateSort context:nil];
    NSLog(@"Sorted:");
    for(NSDate *date in array) {
        NSLog(@"%@", [dateFormatter stringFromDate:date]);
    }    
    
    [dateFormatter release];   
    [array release];
    [pool drain];
    return 0;
}

References:
1. Mac OS Reference Library. NSDate Class Reference.
2. Mac OS Reference Library. NSDateFormatter Class Reference.