1. Date formatting

The java class library contains a Date class whose objects represent a specific instant, accurate to milliseconds.
The representation of time in Java is also a number. The number of milliseconds from 0 o'clock in the standard era 1970.1.1 to a certain time is long.
That is to say, before the zero point of 1970.1.1, it was negative milliseconds and then positive milliseconds.

 	    SimpleDateFormat sdf = new SimpleDateFormat("yyy-MM-dd HH:mm:ss");

		//Converting time to a time string
		Date date = new Date();//Get the current time by default
		Date date2 = new Date(123456456156L);//Pass in long type milliseconds
        System.out.println(sdf.format(date));
        System.out.println(sdf.format(date2));
        
        //Converting a time string to time
        String string = "2019-8-30 11:37:01";
        try {
			Object date3 = sdf.parse(string);
			System.out.println(date3);
			System.out.println(sdf.format(date3));
		} catch (ParseException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

Output results:

2019-08-23 11:40:08
1973-11-30 05:27:36
Fri Aug 30 11:37:01 CST 2019
2019-08-30 11:37:01

The corresponding meaning of each letter:

2. Gregorian Calendar (Calendar)

People's understanding of time is: one year, one month and one day, and the time that the computer knows is a long number. Build a bridge between the two through Calendar.
Be careful:
January: January is 0, February is 1. December is November.
Week: Sunday is 1, Monday is 2. Saturday is 7.
Common methods:

	    Calendar calendar = new GregorianCalendar();//Time object
        calendar.set(2019, Calendar.AUGUST, 30, 22, 10, 23);//Setup time
        System.out.println(calendar.getTime());
        
        calendar.set(Calendar.YEAR, 2018);//Setting the properties of the time
        calendar.set(Calendar.MONTH, 11);
        calendar.set(Calendar.DATE, 23);
        calendar.set(Calendar.HOUR_OF_DAY, 10);//When the following three items are not set, the current time is used by default.
        calendar.set(Calendar.MINUTE, 10);
        calendar.set(Calendar.SECOND, 10);
        System.out.println(calendar.getTime());
        
        System.out.println(calendar.get(Calendar.YEAR));//Years to get the current time
        
        calendar.add(Calendar.HOUR, 10);//Add ten hours to the original time
        System.out.println(calendar.getTime());

Output results:

Fri Aug 30 22:10:23 CST 2019
Sun Dec 23 10:10:10 CST 2018
2018
Sun Dec 23 20:10:10 CST 2018