Difference between 'YYYY' and 'yyyy' in NSDateFormatter

Question

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 calen

Answer 1

dd/MMM/YYYY - e.g.:1 01/Jan/2000; answer : 19/dec/1999
              (see weekly calendar December month last Monday
              suppose leaf year + 1 day)
dd/MMM/yyyy - eg: ordinary - no problem.