The ZoneRules class in Java’s Date-Time API is used to encapsulate the set of rules defining how the zone offset varies for a single time zone.

The information in this class is typically derived from the IANA Time Zone Database (TZDB). The rules model the data traditionally contained in the ‘zic’ compiled data files of information from the TZDB.

An instance of ZoneRules is obtained from a ZoneId using the ZoneId.getRules() method.

Here is a simple example of how to use the ZoneRules class:

package org.kodejava.datetime;

import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.zone.ZoneRules;

public class ZoneRulesExample {
    public static void main(String[] args) {
        // Get ZoneId for "Europe/Paris"
        ZoneId zoneId = ZoneId.of("Europe/Paris");
        System.out.println("ZoneId : " + zoneId);

        // Get ZoneRules associated with the ZoneId
        ZoneRules zoneRules = zoneId.getRules();
        System.out.println("ZoneRules : " + zoneRules);

        // Get the standard offset
        LocalDateTime localDateTime = LocalDateTime.now();
        ZoneOffset offset = zoneRules.getOffset(localDateTime);
        System.out.println("Offset for " + localDateTime + " is: " + offset);
    }
}

Here we are using LocalDateTime.now() to get the current time and the getOffset(LocalDateTime) method on ZoneRules to find the offset for that particular time. The API guarantees immutability and thread-safety of ZoneRules class.

This example will output:

• The ZoneId which will be “Europe/Paris”
• The ZoneRules for the “Europe/Paris” time zone
• The ZoneOffset for the current LocalDateTime. This offset is the difference in time between the “Europe/Paris” time zone and UTC at the time provided.

Output:

ZoneId : Europe/Paris
ZoneRules : ZoneRules[currentStandardOffset=+01:00]
Offset for 2024-01-19T15:55:14.156977 is: +01:00