Date: Work with Moments in Time: Java Tutorial

Java provides a useful and versatile Date class in its java.time package. This class allows us to represent moments in time and dates, and manipulate them however we want with just a few lines of code.

Creating a Date

Creating a date is pretty easy. Simply create a Date object and print it out:

	import java.util.Date;

public class Main {
    public static void main(String[] args) {
        Date now = new Date();
        System.out.println(now.toString());
    }
}

	Sun Dec 15 00:00:00 UTC 2019

Date Formatting

You can format a date object using the SimpleDateFormat class and calling its format() method:

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

public class Main {
    public static void main(String[] args) {
        Date now = new Date();
        SimpleDateFormat format =
 new SimpleDateFormat ("E yyyy-MM-dd 'at' hh:mm:ss a zzz");

        System.out.println(format.format(now));
    }
}

	Sun 2019-12-15 at 00:00:00 AM UTC

If you're curious about what those characters do and if there are any others, here are all the characters that you can use when formatting your dates:

G: Era (AD/BC)
y: Year in four digits (2020)
M: Month (June)
d: Day in month (17)
h: Hour from 1-12
H: Hour from 0-23
m: Minute
s: Second
S: Millisecond
E: Day in week (Friday)
D: Day in year (100)
F: Day of week in month (2nd Friday in June)
w: Week in year (23)
W: Week in month (3)
a: AM/PM
k: Hour in day from 1-24
K: Hour from 0-11
Z: Timezone

Unix Time

Unix time is a concept in computer science that refers to the number of milliseconds that have elapsed since the arbitrarily decided midnight on January 1st, 1970.

Here's how to get that value in Java:

	import java.util.Date;

public class Main {
    public static void main(String[] args) {
        Date now = new Date();
        System.out.println(now.getTime());
    }
}

	1576146483561

Parsing Strings into Dates

When you have a string that represents a date, you can try and parse it into one:

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

public class Main {
    public static void main(String[] args) {
        SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
        String input = "1990-01-01";

        try {
            Date parsed = format.parse(input);
            System.out.println(parsed.toString());
        } catch (ParseException e) {
            // error parsing
        }
    }
}