How To Change Day Of Year In LocalDateTime In Java?

To change or alter the day of year in date-time, you can use the withDayOfYear() method of LocalDateTime class.

This method was added to Java 8 date-time API. So, can be used in Java 8 and higher versions such as Java 11, Java 17, etc.

The withDayOfYear() method returns a copy of the LocalDateTime object after altering the day field(1 to 365-366).

For example, if you want to alter a day of the "2022-03-14T17:28:13.048999208" to "2022-01-10T17:28:13.048999208" then just pass the 10 to the withDayOfYear() method.

Since the year starts on 1 January that's why the result is 10 January of the same year.

Let's understand with the running examples.

Altering day of year in java date-time

/* 
 *  Code example to get local date time with the specified day of year in Java
 */
import java.time.LocalDateTime;
public class JExercise {
	public static void main(String[] args) {		

		// String  date is given
		String strDate = "2022-03-14T17:28:13.048999208";
		// parse the date into date time
		LocalDateTime date = LocalDateTime.parse(strDate);

		// Displaying date and time
		System.out.println("Date : "+date);

		// Get new datetime with altered day
		LocalDateTime newDate = date.withDayOfYear(10);

		// Display new date
		System.out.println("New date : "+newDate);

	}
}

Output:

Date : 2022-03-14T17:28:13.048999208
New date : 2022-01-10T17:28:13.048999208
 

now,  let's have a look at its signature:

public LocalDateTime withDayOfYear(int dayOfYear)

Package Name: java.time;

Class Name: LocalDateTime

Return Value: It returns a copy of LocalDateTime after altering day-of-year.

Parameters: It takes a single int type value.

Exceptions: It throws a DateTimeException if day-of-year value or range(1 to 365-366) is invalid.

Version: Since 1.8

Let's see some more examples.

Alter Day-of-year of current date and time in Java

Here, we are altering day of the current date-time. We used the now() method of LocalDateTime class to get the current date-time and then called the withDayOfYear() method.

We passed the 55 as a value and see it returns date-time of February.

/* 
 *  Code example to get local date time with the specified day of year in Java
 */
import java.time.LocalDateTime;
public class JExercise {
	public static void main(String[] args) {		

		// Current date and time
		LocalDateTime date = LocalDateTime.now();

		// Displaying date and time
		System.out.println("Date : "+date);

		// Get new datetime with altered day
		LocalDateTime newDate = date.withDayOfYear(55);

		// Display new date
		System.out.println("New date : "+newDate);

	}
}

Output:

Date : 2022-03-17T12:10:46.069425754
New date : 2022-02-24T12:10:46.069425754
 

If we pass an invalid range of day-of-year then get the exception like below.

Here, we passed 555 which does not lie in the range(1 to 365-366) that's why the compiler throws an exception.

/* 
 *  Code example to get local date time with the specified day of year in Java
 */
import java.time.LocalDateTime;
public class JExercise {
	public static void main(String[] args) {		

		// Current date and time
		LocalDateTime date = LocalDateTime.now();

		// Displaying date and time
		System.out.println("Date : "+date);

		// Get new datetime with altered day
		LocalDateTime newDate = date.withDayOfYear(555); // invalid range

		// Display new date
		System.out.println("New date : "+newDate);

	}
}

Output:

Date : 2022-03-17T12:11:38.175302293
Exception in thread "main" java.time.DateTimeException: Invalid value for DayOfYear (valid values 1 - 365/366): 555