Understanding the Difference Between yyyy and YYYY in Java SimpleDateFormat

The article discusses a common Java date formatting bug where using YYYY-MM-dd instead of yyyy-MM-dd can cause year rollover issues near year-end.

It explains that y stands for year-of-era, while Y stands for week-based year, which may belong to the next year if the week crosses a year boundary.

Example code:

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

Output for 2019-08-31:

2019-08-31 to yyyy-MM-dd: 2019-08-31
2019-08-31 to YYYY/MM/dd: 2019-08-31

When the date is changed to 2019-12-31, the output becomes:

2019-12-31 to yyyy-MM-dd: 2019-12-31
2019-12-31 to YYYY-MM-dd: 2020-12-31

Developers should use y for calendar dates to avoid such bugs.