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Getting the UTC timestamp in Java

An old Stack Overflow posting suggests that the way to get the UTC timestamp in Java is the following:

Instant.now()   // Capture the current moment in UTC.

Unfortunately this does not work for me. I have a very simple program (reproduced below) which demonstrates different behavior.

On Windows: the time is the local time and it is labeled with the offset with GMT

On Linux: the time is again the local time, and it is labeled correctly for the local timezone

Question: How do we display the UTC timestamp in a Java program?

My sample source code is as follows:

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

public class UTCTimeDisplayer {
    public static void main(String[] args) {
        System.out.println(System.getProperty("os.name"));
        Date currentUtcTime = Date.from(Instant.now());
        System.out.println("Current UTC time is " + currentUtcTime);
    }
}

Windows Output:

C:tmp>java UTCTimeDisplayer
Windows 10
Current UTC time is Fri Jan 22 14:28:59 GMT-06:00 2021

Linux Output:

/tmp> java UTCTimeDisplayer
Linux
Current UTC time is Fri Jan 22 14:31:10 MST 2021

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Answer

The java.util.Date object is not a real date-time object like the modern date-time types; rather, it represents the number of milliseconds since the standard base time known as “the epoch”, namely January 1, 1970, 00:00:00 GMT (or UTC). When you print an object of java.util.Date, its toString method returns the date-time in the JVM’s timezone, calculated from this milliseconds value. If you need to print the date-time in a different timezone, you will need to set the timezone to SimpleDateFormat and obtain the formatted string from it.

I would suggest you simply use Instant.now() which you can convert to other java.time type.

The date-time API of java.util and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern date-time API.

However, if you still want to use java.util.Date, use SimpleDateFormat as mentioned above.

Demo:

import java.text.SimpleDateFormat;
import java.time.Instant;
import java.util.Date;
import java.util.TimeZone;

public class Main {
    public static void main(String[] args) {
        Date currentUtcTime = Date.from(Instant.now());
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss z");
        sdf.setTimeZone(TimeZone.getTimeZone("Etc/UTC"));
        System.out.println("Current UTC time is " + sdf.format(currentUtcTime));
    }
}

Output:

Current UTC time is 2021-01-22 21:53:07 UTC
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