Java 17 Recipes

4-12. Obtaining a Machine Timestamp
 Problem
You need to obtain a machine-based timestamp from the system.
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 Solution
Utilize an Instant class, which represents the start of a nanosecond on the timeline 
based on machine time. In the following example, an Instant class obtains the 
system timestamp. The Instant class is also utilized in other scenarios, such as when 
calculating different dates based on the Instant.
public static void instants(){
Instant timestamp = Instant.now();
System.out.println("The current timestamp: " + timestamp);
//Now minus three days
Instant minusThree = timestamp.minus(3, ChronoUnit.DAYS);
System.out.println("Now minus three days:" + minusThree);
ZonedDateTime atZone = timestamp.atZone(ZoneId.of("GMT"));
System.out.println(atZone);
Instant yesterday = Instant.now().minus(24, ChronoUnit.HOURS);
System.out.println("Yesterday: " + yesterday);
}
Here is the result.
The current timestamp: 2021-10-13T21:06:50.203477900Z
Now minus three days:2021-10-10T21:06:50.203477900Z
2021-10-13T21:06:50.203477900Z[GMT]
Yesterday: 2021-10-12T21:06:50.221430600Z
 How It Works
The Date-Time API introduces a new class named Instant, which represents the start of 
a nanosecond on the timeline in machine-based time. Based on machine time, the value 
in an Instant class counts from EPOCH (January 1, 1970 00:00:00Z). Any values prior to 
the EPOCH are negative, and after the EPOCH the values are positive. The Instant class 
is perfect for obtaining a machine timestamp, as it includes all pertinent date and time 
information to the nanosecond.
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An Instant class is static and immutable, so the now() method can be called to 
obtain the current timestamp. Doing so returns a copy of the current Instant. The 
Instant class also includes conversion and calculation methods. Each returns copies of 
the Instant class or other types. In the solution, the now() method returns the current 
timestamp, and then a couple of examples follow, showing how to perform calculations 
and obtain information on the Instant.
The Instant class is an important new feature in Java 8 because it makes it easy 
to work with the current time and date data. The other date and time classes, such 
as LocalDateTime, are also useful. However, the Instant class is the most accurate 
timestamp because it’s based on nanosecond accuracy.