时间格式问题

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

public class 时间格式 {
    public static void main(String[] args) throws ParseException {
        //获取当前时间
        Date date = new Date();
        //指定日期格式
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
        String s = sdf.format(date);

        //字符串转date类型
        Date dateTestString = sdf.parse(s);

        //获取当前毫秒值
        //https://www.iteye.com/blog/tangmingjie2009-1543166参考博客，方法一会更快点
        //方法 一
        long t1 = System.currentTimeMillis();
        //方法 二
        long t2 = Calendar.getInstance().getTimeInMillis();
        //方法 三
        long t3 = new Date().getTime();

        //毫秒值转换为date
        Date dateTestSeconds = new Date();
        dateTestSeconds.setTime(t2);
        System.out.println(dateTestSeconds);

        //获取当前时间的年月日时分秒
        Calendar calendar = Calendar.getInstance();
        System.out.println(calendar.get(Calendar.YEAR));
        System.out.println(calendar.get(Calendar.MONTH)+1);
        System.out.println(calendar.get(Calendar.DATE));

        //获取指定时间的年月日时分秒
        Calendar calendar1 = Calendar.getInstance();
        calendar1.setTime(sdf.parse("2020/12/21 00:00:00"));
        System.out.println(calendar1.get(Calendar.YEAR));
        System.out.println(calendar1.get(Calendar.MONTH)+1);
        System.out.println(calendar1.get(Calendar.DATE));

        System.out.println(t1);
        System.out.println(t2);
        System.out.println(t3);
        System.out.println(date);
        System.out.println(s);
        System.out.println(dateTestString);
    }
}