每年最后一个跨年的周末，用YYYY会跳到下一年

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;


public class test {
    public static void main(String[] args) {
        Calendar calendar = Calendar.getInstance();
        calendar.set(2021, 11, 31);
        Date strDate = calendar.getTime();
        DateFormat formatUpperCase = new SimpleDateFormat("yyyy-MM-dd");
        System.out.println("2021-12-31 to yyyy-MM-dd: " + formatUpperCase.format(strDate));
        formatUpperCase = new SimpleDateFormat("YYYY-MM-dd");
        System.out.println("2021-12-31 to YYYY/MM/dd: " + formatUpperCase.format(strDate));
    }
}