Why 'YYYY-MM-dd' Gives the Wrong Year in Java – The Week‑Based Year Pitfall

The article discusses a common bug where using the pattern "YYYY-MM-dd" with Java's SimpleDateFormat yields an unexpected year for dates near the end of the year.

Test Setup

Two formatters are created: one with the problematic pattern "YYYY-MM-dd" and another with the correct pattern "yyyy-MM-dd". Both are used to format December 25, 2021 (Saturday) and December 26, 2021 (Sunday).

public class Tests {

    @Test
    public void test() throws Exception {
        SimpleDateFormat df1 = new SimpleDateFormat("YYYY-MM-dd");
        SimpleDateFormat df2 = new SimpleDateFormat("yyyy-MM-dd");

        Calendar c = Calendar.getInstance();
        c.set(Calendar.YEAR, 2021);
        c.set(Calendar.MONTH, 11);
        // 2021-12-25 Saturday
        c.set(Calendar.DATE, 25);
        System.out.println("YYYY-MM-dd = " + df1.format(c.getTime()));
        System.out.println("yyyy-MM-dd = " + df2.format(c.getTime()));
        System.out.println("========================");
        // 2021-12-26 Sunday
        c.set(Calendar.DATE, 26);
        System.out.println("YYYY-MM-dd = " + df1.format(c.getTime()));
        System.out.println("yyyy-MM-dd = " + df2.format(c.getTime()));
    }
}

Running the test shows that for December 25 both patterns output 2021‑12‑25, but for December 26 the "YYYY-MM-dd" pattern outputs 2022‑12‑26 while the "yyyy-MM-dd" pattern correctly outputs 2021‑12‑26.

Reason

The symbol "Y" represents the week‑based year, which belongs to the year of the week that the date falls in. Since weeks start on Sunday, a Sunday that belongs to the first week of the next year causes the week‑based year to advance, so "YYYY" on 2021‑12‑26 yields 2022.

Using "yyyy" (calendar year) avoids this issue.