Understanding the Difference Between yyyy and YYYY in Java Date Formatting

During a holiday test the author noticed an app showing the date 2020‑12‑30 instead of the expected 2019‑12‑30, which turned out to be caused by using the wrong date pattern in Java.

The following Java program creates a Calendar set to 2019‑08‑31, formats the date with SimpleDateFormat using both yyyy-MM-dd and YYYY-MM-dd , and prints the results:

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));
    }
}

Running the program yields:

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

The discrepancy arises because yyyy represents the calendar year (year‑of‑era), while YYYY denotes the week‑based year; if the week containing December 31 belongs to the next year, YYYY will output that next year.

This small but common pitfall can confuse users, so developers should prefer yyyy for ordinary year formatting to avoid such bugs.