default Instant toInstant()

// Java program to demonstrate
// ChronoZonedDateTime.toInstant() method
  
import java.time.*;
import java.time.chrono.*;
import java.time.chrono.*;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // create ChronoZonedDateTime object
        ChronoZonedDateTime time
            = ZonedDateTime
                  .parse(
                      "2018-12-06T19:21:12.123+05:30[Asia/Calcutta]");
  
        // print ChronoZonedDateTime
        System.out.println("ChronoZonedDateTime: "
                           + time);
  
        // print result
        System.out.println("Instant: "
                           + time.toInstant());
    }
}

ChronoZonedDateTime: 2018-12-06T19:21:12.123+05:30[Asia/Calcutta]
Instant: 2018-12-06T13:51:12.123Z

// Java program to demonstrate
// ChronoZonedDateTime.toInstant() method
  
import java.time.*;
import java.time.chrono.*;
import java.time.chrono.*;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // create ChronoZonedDateTime object
        ChronoZonedDateTime time
            = ZonedDateTime.parse(
                "1918-10-25T23:12:38.543+02:00[Europe/Paris]");
  
        // print ChronoZonedDateTime
        System.out.println("ChronoZonedDateTime: "
                           + time);
  
        // print result
        System.out.println("Instant: "
                           + time.toInstant());
    }
}

ChronoZonedDateTime: 1918-10-25T23:12:38.543Z[Europe/Paris]
Instant: 1918-10-25T23:12:38.543Z