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

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.

from Apple Docs