Why Using “YYYY” in SimpleDateFormat Breaks Dates Around New Year (and How to Fix It)

Reproducing the Bug

The following Java program uses SimpleDateFormat with the pattern "YYYY-MM-dd HH:mm:ss" to parse the string "2020-1-1 13:12:12" and prints the parsed Date and the formatted result:

public class DateFormatBug {
    public static void main(String[] args) throws ParseException {
        printBugDate();
    }

    private static void printBugDate() throws ParseException {
        SimpleDateFormat sdf = new SimpleDateFormat("YYYY-MM-dd HH:mm:ss");
        Date date = sdf.parse("2020-1-1 13:12:12");
        System.out.println(date);
        String dateStr = sdf.format(date);
        System.out.println(dateStr);
    }
}

Running this code prints:

Sun Dec 29 13:12:12 CST 2019
2020-12-29 13:12:12

The parsed date is in 2019, while the formatted string shows December 29, 2020 – an incorrect result.

Extended Example

Parsing "2019-12-31 13:12:12" with the same pattern yields another wrong year:

private static void printBugDateExtend() throws ParseException {
    SimpleDateFormat sdf = new SimpleDateFormat("YYYY-MM-dd HH:mm:ss");
    Date date = sdf.parse("2019-12-31 13:12:12");
    System.out.println(date);
    String dateStr = sdf.format(date);
    System.out.println(dateStr);
}

Sun Dec 30 13:12:12 CST 2018
2019-12-30 13:12:12

Both examples demonstrate that the year component is shifted.

Why It Happens

In SimpleDateFormat the pattern letter Y does **not** represent the calendar year; it denotes the *week‑based year* (ISO‑8601 week year). The week‑based year can differ from the calendar year at the start or end of a year, especially when the first week does not contain at least four days of the new year. This explains the off‑by‑one behavior for dates near New Year’s.

Official documentation (Javadoc) describes this behavior: https://docs.oracle.com/javase/8/docs/api/java/time/temporal/WeekFields.html

Fixing the Issue

Use the lowercase y pattern letter to represent the calendar year. Updating the format strings eliminates the bug:

private static void printDate() throws ParseException {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Date date = sdf.parse("2020-1-1 13:12:12");
    System.out.println(date);
    String dateStr = sdf.format(date);
    System.out.println(dateStr);
}

private static void printDateExtend() throws ParseException {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Date date = sdf.parse("2019-12-31 13:12:12");
    System.out.println(date);
    String dateStr = sdf.format(date);
    System.out.println(dateStr);
}

Running the corrected code yields the expected output:

Wed Jan 01 13:12:12 CST 2020
2020-01-01 13:12:12
Tue Dec 31 13:12:12 CST 2019
2019-12-31 13:12:12

Takeaway

Never use the uppercase Y for the year in date‑time patterns unless the week‑based year is explicitly required. Prefer lowercase y to avoid subtle cross‑year bugs in production systems.