Why Does Using 'YYYY' in Java Date Formatting Show the Wrong Year?

When using an app I noticed a bug where a date displayed as 2020‑12‑30 instead of the intended 2019‑12‑30, caused by using the wrong date pattern in Java.

Below is a simple Java example that formats a Calendar date using both yyyy and YYYY patterns.

public class DateTest {
    public static void main(String[] args) {
        Calendar calendar = Calendar.getInstance();
        calendar.set(2019, Calendar.AUGUST, 31);
        Date date = calendar.getTime();
        SimpleDateFormat fmt = new SimpleDateFormat("yyyy-MM-dd");
        System.out.println("2019-08-31 to yyyy-MM-dd: " + fmt.format(date));
        fmt = new SimpleDateFormat("YYYY-MM-dd");
        System.out.println("2019-08-31 to YYYY/MM/dd: " + fmt.format(date));

        calendar.set(2019, Calendar.DECEMBER, 31);
        date = calendar.getTime();
        fmt = new SimpleDateFormat("yyyy-MM-dd");
        System.out.println("2019-12-31 to yyyy-MM-dd: " + fmt.format(date));
        fmt = new SimpleDateFormat("YYYY-MM-dd");
        System.out.println("2019-12-31 to YYYY-MM-dd: " + fmt.format(date));
    }
}

The output shows that for dates in the last week of the year, YYYY may roll over to the next year because it represents the week‑based year, while yyyy represents the calendar year.

Understanding this subtle difference prevents bugs where users see an unexpected year, especially around New Year's week.