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Objective-C - Objective C - NSDate
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  Creating an NSDate

The NSDate class provides methods for creating NSDate objects corresponding to a given date and time. An NSDate can be initialized using the designated initializer, which:

Returns an NSDate object initialized relative to 00:00:00 UTC on 1 January 2001 by a given number of seconds.

NSDate *date = [[NSDate alloc] initWithTimeIntervalSinceReferenceDate:100.0];

NSDate also provides an easy way to create an NSDate equal to the current date and time:

NSDate *now = [NSDate date];

It is also possible to create an NSDate a given amount of seconds from the current date and time:

NSDate *tenSecondsFromNow = [NSDate dateWithTimeIntervalSinceNow:10.0];
  Date Comparison

There are 4 methods for comparing NSDates in Objective-C:

  • - (BOOL)isEqualToDate:(NSDate *)anotherDate
  • - (NSDate *)earlierDate:(NSDate *)anotherDate
  • - (NSDate *)laterDate:(NSDate *)anotherDate
  • - (NSComparisonResult)compare:(NSDate *)anotherDate

Consider the following example using 2 dates, NSDate date1 = July 7, 2016 and NSDate date2 = July 2, 2016:

NSDateComponents *comps1 = [[NSDateComponents alloc]init];
comps.year = 2016;
comps.month = 7;
comps.day = 7;

NSDateComponents *comps2 = [[NSDateComponents alloc]init];
    comps.year = 2016;
    comps.month = 7;
    comps.day = 2;

NSDate* date1 = [calendar dateFromComponents:comps1]; //Initialized as July 7, 2016 
NSDate* date2 = [calendar dateFromComponents:comps2]; //Initialized as July 2, 2016

Now that the NSDates are created, they can be compared:

if ([date1 isEqualToDate:date2]) {
    //Here it returns false, as both dates are not equal
}

We can also use the earlierDate: and laterDate: methods of the NSDate class:

NSDate *earlierDate = [date1 earlierDate:date2];//Returns the earlier of 2 dates. Here earlierDate will equal date2.
NSDate *laterDate = [date1 laterDate:date2];//Returns the later of 2 dates. Here laterDate will equal date1.

Lastly, we can use NSDate's compare: method:

NSComparisonResult result = [date1 compare:date2];
    if (result == NSOrderedAscending) {
        //Fails
        //Comes here if date1 is earlier than date2. In our case it will not come here.
    }else if (result == NSOrderedSame){
        //Fails
        //Comes here if date1 is the same as date2. In our case it will not come here.
    }else{//NSOrderedDescending
        
        //Succeeds
        //Comes here if date1 is later than date2. In our case it will come here
    }
  Convert NSDate that is composed from hour and minute (only) to a full NSDate

There are many cases when one has created an NSDate from only an hour and minute format, i.e: 08:12

The downside for this situation is that your NSDate is almost completely "naked" and what you need to do is to create: day, month, year, second and time zone in order to this object to "play along" with other NSDate types.

For the sake of the example let's say that hourAndMinute is the NSDate type that is composed from hour and minute format:

NSDateComponents *hourAndMintuteComponents = [calendar components:NSCalendarUnitHour | NSCalendarUnitMinute
                                                         fromDate:hourAndMinute];
NSDateComponents *componentsOfDate = [[NSCalendar currentCalendar] components:NSCalendarUnitDay | NSCalendarUnitMonth | NSCalendarUnitYear
                                                                     fromDate:[NSDate date]];

NSDateComponents *components = [[NSDateComponents alloc] init];
[components setDay: componentsOfDate.day];
[components setMonth: componentsOfDate.month];
[components setYear: componentsOfDate.year];
[components setHour: [hourAndMintuteComponents hour]];
[components setMinute: [hourAndMintuteComponents minute]];
[components setSecond: 0];
[calendar setTimeZone: [NSTimeZone defaultTimeZone]];

NSDate *yourFullNSDateObject = [calendar dateFromComponents:components];

Now your object is the total opposite of being "naked".
  Converting NSDate to NSString

If ww have NSDate object, and we want to convert it into NSString. There are different types of Date strings. How we can do that?, It is very simple. Just 3 steps.

  1. Create NSDateFormatter Object

   NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
  1. Set the date format in which you want your string.

   dateFormatter.dateFormat = @"yyyy-MM-dd 'at' HH:mm";
  • Now, get the formatted string 
     NSDate *date = [NSDate date]; // your NSDate object
 NSString *dateString = [dateFormatter stringFromDate:date];

    • This will give output something like this: 2001-01-02 at 13:00

    Note:

    Creating an NSDateFormatter instance is an expensive operation, so it is recommended to create it once and reuse when possible.

    Remarks

    NSDate is a very simple value type, representing one exact moment in universal time. It does not contain information about time zones, daylight saving time, calendars, or locale.

    NSDate is really only an immutable wrapper around an NSTimeInterval which is a double. There is no mutable subclass, as with some other value types in Foundation.

    		 

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    https://develop.horje.com/learn/1434/reference

     

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