Understanding the Difference Between yyyy and YYYY in Java Date Formatting

In the introductory section the author describes encountering a bug where an app displayed the date 2020‑12‑30 instead of the expected 2019‑12‑30, which was caused by using the wrong date pattern.

The author then presents a Java example that sets a calendar to 2019‑08‑31 and formats the date using both yyyy-MM-dd and YYYY-MM-dd patterns.

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 this code produces the following output, showing that both patterns give the same result for this date:

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 differs:

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

The discrepancy is due to the meaning of the pattern letters: y stands for the calendar year (year‑of‑era), while Y represents the week‑based year. If the week containing December 31 spans into the next year, Y will report the next year.

The article concludes by reminding developers to use the correct pattern ( yyyy ) for calendar years and to be aware of this subtle pitfall that can easily slip through testing.