What's the Difference Between 'Y' and 'y' in Java's SimpleDateFormat for Year Representation?

Understanding the Disparity between 'Y' and 'y' in SimpleDateFormat

SimpleDateFormat, a format for displaying dates in Java, offers a range of formatting options signified by single-character specifiers. Among these, 'Y' and 'y' represent the year, but they exhibit an intriguing difference.

'Y' denotes the week year, which aligns with the week of the year cycle. This implies that all weeks within the first and last weeks of the cycle share the same week year value. Consequently, the first and last days of a week year might belong to different calendar years.

On the other hand, 'y' represents the calendar year. It always follows the standard Gregorian calendar, starting on January 1st and ending on December 31st.

The distinction between 'Y' and 'y' can be illustrated through an example. If the week year calculations adopt the ISO 8601 standard (starting Monday with at least four days in the first week), January 1st, 1998 falls on a Thursday. This means that the first week of 1998 starts on December 29th, 1997, extending to January 4th, 1998. As a result, the last three days of 1997 belong to week year 1998. However, if the week year starts on Sunday, the first week of 1998 would begin on January 4th, 1998, and the first three days of 1998 would fall under week 53 of 1997.

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