In this article, we will learn how to convert Instant to GregorianCalendar and vice-versa in Java 1.8 version

Instant to GregorianCalendar conversion & vice-versa :

• There are 3 new methods introduced in Java 1.8 version for GregorianCalendar class, those are
  1. from(ZonedDateTime) – This static method obtains an instance of GregorianCalendar with the default locale from a ZonedDateTime object
  2. toZonedDateTime() – This method converts invoking GregorianCalendar object to a ZonedDateTime that represents the same point on the time-line as that of GregorianCalendar object
  3. getCalendarType() – Returns "gregory" as the calendar type

1. Convert Instant to GregorianCalendar :

• GregorianCalendar.from() method accepts ZonedDateTime as input-argument and returns GregorianCalendar
  • First, convert Instant to ZonedDateTime using atZone() method of Instant passing ZoneId as input–argument which will return ZonedDateTime
  • And then pass converted ZonedDateTime to GregorianCalendar.from() method which returns GregorianCalendar
• Converted GregorianCalendar will have,
  • Date & Time & Zone/Offset parts shifted to that particular Zone
• In short, Instant -> ZonedDateTime -> GregorianCalendar
• Lets see an example for conversion of Instant to GregorianCalendar in the below illustration

ConvertInstantToGregorianCalendar.java

package in.bench.resources.java8.instant.examples;

import java.time.Instant;
import java.time.ZoneId;
import java.util.GregorianCalendar;

public class ConvertInstantToGregorianCalendar {

	public static void main(String[] args) {

		// 1. get instantaneous moment at UTC/GMT
		Instant instant = Instant.now();
		System.out.println("Current Instant at UTC/GMT is :- \n" 
				+ instant);


		// 2. convert Instant -> ZonedDateTime -> GregorianCalendar 
		GregorianCalendar gregorianCalendar = GregorianCalendar
				.from(instant.atZone(ZoneId.systemDefault()));


		// 3. print to console
		System.out.print("\nConversion of Instant to GregorianCalendar is :- \n"
				+ gregorianCalendar.getTime());
	}
}

Output:

Current Instant at UTC/GMT is :- 
2022-08-21T05:45:26.395963100Z

Conversion of Instant to GregorianCalendar is :- 
Sun Aug 21 11:15:26 IST 2022

2. Convert GregorianCalendar to an Instant :

• Instantiate GregorianCalendar object for conversion to an Instant
• Conversion steps –
  • First, convert GregorianCalendar to ZonedDateTime using toZonedDateTime() method
  • And then convert ZonedDateTime to an Instant using toInstant() method
• Converted Instant will have,
  • Date & Time & Zone/Offset parts represents current Instant date/time at GMT/UTC
• In short, GregorianCalendar -> ZonedDateTime -> Instant
• Lets see an example for conversion of GregorianCalendar to an Instant in the below illustration

ConvertGregorianCalendarToInstant.java

package in.bench.resources.java8.instant.examples;

import java.time.Instant;
import java.util.GregorianCalendar;

public class ConvertGregorianCalendarToInstant {

	public static void main(String[] args) {

		// 1. Instantiate GregorianCalendar
		GregorianCalendar gregorianCalendar = new GregorianCalendar();
		System.out.println("Current Date/Time is :- \n" 
				+ gregorianCalendar.getTime());


		// 2. convert GregorianCalendar -> ZonedDateTime -> Instant
		Instant instant = gregorianCalendar
				.toZonedDateTime()
				.toInstant();


		// 3. print to console
		System.out.print("\nConversion of GregorianCalendar to an Instant is :- \n"
				+ instant);
	}
}

Output:

Current Date/Time is :- 
Sun Aug 21 11:15:11 IST 2022

Conversion of GregorianCalendar to an Instant is :- 
2022-08-21T05:45:11.522Z

Related Articles:

• Java 8 – Instant with method details and examples
• Java 8 – How to get instantaneous moment at UTC/GMT using Instant ?
• Java 8 – How to form an Instant passing Seconds and Nanoseconds fields ?
• Java 8 – How to get Seconds and Nanoseconds from an Instant ?
• Java 8 – How to parse Instant in String form ?
• Java 8 – How to convert Instant to LocalDate ?
• Java 8 – How to convert Instant to LocalTime ?
• Java 8 – How to convert Instant to LocalDateTime ?
• Java 8 – How to convert Instant to ZonedDateTime ?
• Java 8 – How to convert Instant to an OffsetDateTime ?
• Java 8 – How to convert Instant to number of Seconds and vice-versa ?
• Java 8 – How to convert Instant to number of Milliseconds and vice-versa ?
• Java 8 – How to convert Instant to java.util.Date and vice-versa ?
• Java 8 – How to convert Instant to java.sql.Timestamp and vice-versa ?
• Java 8 – How to convert Instant to Calendar and vice-versa ?
• Java 8 – How to convert Instant to GregorianCalendar and vice-versa ?
• Java 8 – How to convert Instant to XMLGregorianCalendar and vice-versa ?
• Java 8 – How to convert java.util.Date to an Instant in different ways ?
• Java 8 – How to add Second, Millisecond and Nanosecond to an Instant ?
• Java 8 – How to subtract Second, Millisecond and Nanosecond from an Instant ?
• Java 8 – How to check whether an Instant is Before another Instant ?
• Java 8 – How to check whether an Instant is After another Instant ?
• Java 8 – How to compare two Instant instances ?
• Java 8 – How to find difference between two Instant instances using Duration ?
• Java 9 – Find difference between two Instant instances upto nanosecond precision ?
• Java 9 – How to convert Instant to LocalDate using ofInstant() method ?
• Java 9 – How to convert Instant to LocalTime using ofInstant() method ?
• More Java 8 Date/Time API examples

References:

• https://docs.oracle.com/javase/8/docs/api/java/util/GregorianCalendar.html
• https://docs.oracle.com/javase/8/docs/api/java/time/Instant.html
• https://docs.oracle.com/javase/8/docs/api/java/time/ZonedDateTime.html
• https://docs.oracle.com/javase/8/docs/api/java/time/ZoneId.html
• https://docs.oracle.com/javase/8/docs/api/java/time/ZoneOffset.html
• https://docs.oracle.com/javase/8/docs/api/java/util/Date.html
• https://docs.oracle.com/javase/8/docs/api/java/util/Calendar.html
• https://docs.oracle.com/javase/8/docs/api/java/sql/Timestamp.html

Happy Coding !!
Happy Learning !!