The pit of Java SimpleDateFormat expression

Today is December 30, 2019. I logged into our online server and found that the log became "localhost_access_log.2020-12-30.log", which was a whole year away. All the monitoring graphs based on this log were scrapped, and I was panicked. I quickly debugged and found the SimpleDateFormat pit.
Let me talk about our environment first: springboot2.1.7 && built-in tomcat 9.0.22, and configured access log for tomcat, the specific configuration is as follows

server.tomcat.accesslog.enabled=true
server.tomcat.accesslog.pattern=********
server.tomcat.accesslog.prefix=localhost_access_log
server.tomcat.accesslog.suffix=.log
server.tomcat.accesslog.file-date-format=.YYYY-MM-dd
server.tomcat.accesslog.rotate=true
server.tomcat.accesslog.directory=logs
server.tomcat.basedir=./

Screenshot of the online log.
According to the log, debug tomcat all the way to initialize the application, and it turns out that the file in AccessLogValve

    @Override
    protected synchronized void startInternal() throws LifecycleException {

        // Initialize the Date formatters
        String format = getFileDateFormat();
        fileDateFormatter = new SimpleDateFormat(format, Locale.US);
        fileDateFormatter.setTimeZone(TimeZone.getDefault());
        dateStamp = fileDateFormatter.format(new Date(System.currentTimeMillis())); // **出问题的地方**
        if (rotatable && renameOnRotate) {
            restore();
        }
        open();

        super.startInternal();
    }

Then manually wrote the test class, the problem reappeared

public static void main(String[] args) {
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("YYYY-MM-dd", Locale.US);
        long time = System.currentTimeMillis();
        time = 1577601170000l; // 2019-12-29 14:32:50
        String dateStamp = simpleDateFormat.format(new Date(time));
        System.out.println(dateStamp);
    }

Later, I looked at the jdk official website and found that there was a problem
with the time formatting expression . There are two Year and Week years in the figure above. Year is the year on the normal calendar, the latter refers to the official document . Simply put, December 30, 2019 is exactly the first day of the first week of the 2020 Week-year. So the result of "YYYY" is 2019.
After modifying the code, SimpleDateFormat df = new SimpleDateFormat(".yyyy.MM.dd");, you can get the correct result,
similar to "D" and "d". Therefore, "y" and "d" should be used in most cases.
As follows:

public static void main(String[] args) {
//        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("YYYY-MM-dd", Locale.US);
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd", Locale.US);
        long time = System.currentTimeMillis();
        time = 1577601170000l; // 2019-12-29 14:32:50
        String dateStamp = simpleDateFormat.format(new Date(time));
        System.out.println(dateStamp);
    }

This detail is usually not paid attention to. Fortunately, it only appears on the log name today, otherwise the impact cannot be assessed.