Understanding the Difference Between yyyy and YYYY in Java Date Formatting

Hello everyone, I'm Architecture Jun, an architect who writes code and poetry.

Preface: While traveling during the New Year holiday, I noticed an app displaying the wrong year—showing 2020-12-30 instead of 2019-12-30—due to a front‑end oversight.

Because such mistakes can mislead users, I share the issue I encountered when I previously studied Java.

Code example:

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 prints:

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

If the date is changed to December 31:

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

The problem appears because 'y' denotes year‑of‑era, while 'Y' denotes week‑based‑year; when the week spans the new year, 'Y' can belong to the next year.

Although this is a small detail, many developers fall into this pitfall; record it and share if you find it useful. I wish you bug‑free coding.