Java Unix time

last modified January 10, 2023

Java Unix time tutorial shows how to compute Unix time in Java.

Unix time (also known as POSIX time or epoch time), is a system for describing a point in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970, minus the number of leap seconds that have taken place since then.

Unix time is widely used on Unix-like operating systems but also in many other computing systems and file formats. It is often used by webmasters because a Unix timestamp can represent all time zones at once.

Unix timestamps should be stored as long numbers; if they are store as Java int values, then this leads to a 2038 year problem. 32-bit variables cannot encode times after 03:14:07 UTC on 19 January 2038.

$ date +%s
1517213809

We can use the date command to determine Unix time on Linux. Unix time can be determined on the https://www.unixtimestamp.com/.

Java Unix time example

The following example computes the Unix time.

package com.zetcode;

import java.time.Instant;
import java.util.Date;

public class JavaUnixTimeEx {

    public static void main(String[] args) {

        long ut1 = Instant.now().getEpochSecond();
        System.out.println(ut1);

        long ut2 = System.currentTimeMillis() / 1000L;
        System.out.println(ut2);

        Date now = new Date();
        long ut3 = now.getTime() / 1000L;
        System.out.println(ut3);
    }
}

There are three basic ways to compute Unix time in Java.

long ut1 = Instant.now().getEpochSecond();
System.out.println(ut1);

Since Java 8, it is possible to use Instant and its getEpochSecond to compute the Unix time.

long ut2 = System.currentTimeMillis() / 1000L;
System.out.println(ut2);

Here we compute the Unix time with System.currentTimeMillis method. We need to transform milliseconds to seconds.

Date now = new Date();
long ut3 = now.getTime() / 1000L;
System.out.println(ut3);

We can also use the old Date class to compute the Unix time.

In this article, we have shown how to compute Unix time in Java.

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