Epoch in Java

A common requirement in programs is getting the number of seconds, milliseconds, or nanoseconds since the Unix epoch. Here’s how to do it in Java.

import java.time.Instant;

public class Epoch {
    public static void main(String[] args) {
        // Use Instant.now() to get the current time
        Instant now = Instant.now();
        System.out.println(now);

        // Get elapsed time since the Unix epoch in seconds,
        // milliseconds or nanoseconds
        System.out.println(now.getEpochSecond());
        System.out.println(now.toEpochMilli());
        System.out.println(now.getNano());

        // You can also convert integer seconds or nanoseconds
        // since the epoch into the corresponding Instant
        System.out.println(Instant.ofEpochSecond(now.getEpochSecond()));
        System.out.println(Instant.ofEpochSecond(0, now.getNano()));
    }
}

To run the program:

$ javac Epoch.java
$ java Epoch
2023-05-10T12:34:56.789Z
1683723296
1683723296789
789000000
2023-05-10T12:34:56Z
1970-01-01T00:00:00.789Z

In this Java example, we use the Instant class from the java.time package to work with instants on the timeline. The Instant.now() method gives us the current time, and we can use various methods to get the time in different formats:

• getEpochSecond() returns the number of seconds from the epoch
• toEpochMilli() returns the number of milliseconds from the epoch
• getNano() returns the nanosecond-of-second

We can also create Instant objects from epoch seconds or nanoseconds using Instant.ofEpochSecond().

Next, we’ll look at another time-related task: time parsing and formatting.