ZonedDateTime withDayOfMonth() method in Java with Examples

withDayOfMonth() method of a ZonedDateTime class used to alter day-of-month in this ZonedDateTime and return a copy of ZonedDateTime after this operation.This method operates on the local time-line, changing the day-of-month of the local date-time and after this operation local date-time is converted back to a ZonedDateTime, using the zone ID to obtain the offset. When converting back to ZonedDateTime, if the local date-time is in an overlap, then the offset will be retained if possible, otherwise, the earlier offset will be used. This instance is immutable and unaffected by this method call.

Syntax:

public ZonedDateTime withDayOfMonth(int dayOfMonth)

Parameters: This method accepts a single parameter dayOfMonth which represents the day-of-month to set in the result, from 1 to 28-31.

Return value: This method returns a ZonedDateTime based on this date-time with the requested day.

Exception: This method throws DateTimeException if the day-of-month value is invalid, or if the day-of-month is invalid for the month-year.

Below programs illustrate the withDayOfMonth() method:
Program 1:

// Java program to demonstrate
// ZonedDateTime.withDayOfMonth() method

import java.time.*;

public class GFG {
    public static void main(String[] args)
    {

        // create a ZonedDateTime object
        ZonedDateTime zoneddatetime
            = ZonedDateTime.parse(
                "2018-12-06T19:21:12.123+05:30[Asia/Calcutta]");

        // print instance
        System.out.println("ZonedDateTime before"
                           + " alter dayOfMonth: "
                           + zoneddatetime);

        // alter dayOfMonth to 5th
        ZonedDateTime returnvalue
            = zoneddatetime.withDayOfMonth(5);

        // print result
        System.out.println("ZonedDateTime after "
                           + "alter dayOfMonth: "
                           + returnvalue);
    }
}

Output:

ZonedDateTime before alter dayOfMonth: 2018-12-06T19:21:12.123+05:30[Asia/Calcutta]
ZonedDateTime after alter dayOfMonth: 2018-12-05T19:21:12.123+05:30[Asia/Calcutta]

Program 2:

// Java program to demonstrate
// ZonedDateTime.withDayOfMonth() method

import java.time.*;

public class GFG {
    public static void main(String[] args)
    {

        // create a ZonedDateTime object
        ZonedDateTime zoneddatetime
            = ZonedDateTime.parse(
                "2018-10-25T23:12:31.123+02:00[Europe/Paris]");

        // print instance
        System.out.println("ZonedDateTime before"
                           + " alter dayOfMonth: "
                           + zoneddatetime);

        // alter dayOfMonth to 20
        ZonedDateTime returnvalue
            = zoneddatetime.withDayOfMonth(20);

        // print result
        System.out.println("ZonedDateTime after "
                           + "alter dayOfMonth: "
                           + returnvalue);
    }
}

Output:

ZonedDateTime before alter dayOfMonth: 2018-10-25T23:12:31.123+02:00[Europe/Paris]
ZonedDateTime after alter dayOfMonth: 2018-10-20T23:12:31.123+02:00[Europe/Paris]

Reference: https://docs.oracle.com/javase/10/docs/api/java/time/ZonedDateTime.html#withDayOfMonth(int)