Difference between 'YYYY' and 'aaaa' in NSDateFormatter

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What is exact difference between 'YYYY' and 'yyyy'. I read in this link, it states that

A common mistake is to use YYYY. yyyy specifies the calendar year whereas YYYY specifies the year (of “Week of Year”), used in the ISO year-week calendar. In most cases, yyyy and YYYY yield the same number, however they may be different. Typically you should use the calendar year.

But when I try to use

NSString *stringDate = @"Feb 28, 2013 05:30pm";
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"MMM dd, yyyy hh:mma"];
NSDate *date=[dateFormatter dateFromString:stringDate];
NSLog(@"Date 1 : %@",date); //2013-02-28 12:00:00 +0000

NSString *stringDatee = @"Feb 28, 2013 05:30pm";
NSDateFormatter *dateFormatterr = [[NSDateFormatter alloc] init];
[dateFormatterr setDateFormat:@"MMM dd, YYYY hh:mma"];
NSDate *datee=[dateFormatterr dateFromString:stringDatee];
NSLog(@"Date 2 : %@",datee); //2013-01-05 12:00:00 +0000

NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setDateFormat:@"MMM dd, YYYY hh:mma"];
NSString *dateString = [dateFormat stringFromDate:datee];
NSLog(@"date 3 : %@", dateString); //Jan 05, 2013 05:30PM

As here, result to date and datee different, which I understood, but why result of date 2 and date 3 are different? As I am creating date from string and reversing same to string again, but output mismatches?

Has anybody knows reason about same?. Though it specifies week of year, still I should get result same.

Thanks..

EDIT :-

If I code

NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setDateFormat:@"MMM dd, YYYY hh:mma"];
NSString *dateString = [dateFormatterr stringFromDate:[NSDate date]];
NSLog(@"date: %@", dateString); //Feb 28, 2013 04:37PM

If results me proper result, but same which I pass as string to date I get 2013-01-05 12:00:00 +0000, check date 2 of NSLog, Strange result, why?


Also when using a date format string using the correct format is important.

@"YYYY" is week-based calendar year.

@"yyyy" is ordinary calendar year.

You can go through the whole blog, its a good to give it a look

http://realmacsoftware.com/blog/working-with-date-and-time