An instant in the datetime continuum specified as a number of milliseconds from 1970-01-01T00:00Z. Some classes that represent an instance in the Joda-Time library includes the Instant, DateTime, DateMidnight and MutableDateTime classes.

package org.kodejava.joda;

import org.joda.time.DateTime;
import org.joda.time.Instant;

public class InstantDemo {
    public static void main(String[] args) {
        // An instant in the datetime continuum specified as
        // a number of milliseconds from 1970-01-01T00:00Z.
        //
        // The declaration below creates 1 seconds instant from
        // 1970.
        Instant instant = new Instant(1000);

        // Get a new copy of instant with 500 duration added.
        instant = instant.plus(500);

        // Get a new copy of instant with 250 duration taken away.
        instant = instant.minus(250);
        System.out.println("Milliseconds = " + instant.getMillis());

        // Creating an instant that represent the current date.
        DateTime dateTime = new DateTime();
        System.out.println("Date Time = " + dateTime);

        // Creating an instant of a specific date and time.
        DateTime independenceDay = new DateTime(1945, 8, 17, 0, 0, 0);
        System.out.println("Independence Day = " + independenceDay);
    }
}

Here is the result of our program:

Date Time = 2021-10-29T08:58:30.011+08:00
Independence Day = 1945-08-17T00:00:00.000+09:00

Maven Dependencies

<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.12.7</version>
</dependency>

This example demonstrate how to use the ISODateTimeFormat class to format the date time information in Joda-Time.

package org.kodejava.joda;

import org.joda.time.DateTime;
import org.joda.time.format.ISODateTimeFormat;

public class ISODateTimeFormatDemo {
    public static void main(String[] args) {
        DateTime dateTime = DateTime.now();

        // Returns a basic formatter for a full date as four digit
        // year, two-digit month of year, and two digit day of
        // month yyyyMMdd.
        System.out.println(dateTime.toString(
                ISODateTimeFormat.basicDate()));
        System.out.println(dateTime.toString(
                ISODateTimeFormat.basicDateTime()));
        System.out.println(dateTime.toString(
                ISODateTimeFormat.basicDateTimeNoMillis()));

        // Returns a formatter for a full ordinal date, using a four
        // digit year and three digit dayOfYear yyyyDDD.
        System.out.println(dateTime.toString(
                ISODateTimeFormat.basicOrdinalDate()));

        // Returns a basic formatter for a full date as four digit
        // weekyear, two-digit week of weekyear, and one digit day
        // of week xxxx'W'wwe
        System.out.println(dateTime.toString(
                ISODateTimeFormat.basicWeekDate()));
        System.out.println(dateTime.toString(
                ISODateTimeFormat.basicWeekDateTime()));
    }
}

The result of the code above is printed below:

20211029
20211029T091631.884+0800
20211029T091631+0800
2021302
2021W435
2021W435T091631.884+0800

Maven Dependencies

<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.12.7</version>
</dependency>

The code below show you how to use the Months class to calculate the different in months between two dates.

package org.kodejava.joda;

import org.joda.time.DateMidnight;
import org.joda.time.LocalDate;
import org.joda.time.Months;

import java.util.Date;

public class MonthsBetweenDemo {
    public static void main(String[] args) {
        // Creates an instance of start and end dates.
        DateMidnight start = new DateMidnight("2021-01-01");
        DateMidnight end = new DateMidnight(new Date());

        // Get months between these dates.
        int months = Months.monthsBetween(start, end).getMonths();

        // Print the result.
        System.out.println("Months between " +
                start.toString("yyyy-MM-dd") + " and " +
                end.toString("yyyy-MM-dd") + " = " +
                months + " month(s)");

        // Using LocalDate object.
        LocalDate date1 = LocalDate.parse("2021-03-01");
        LocalDate date2 = LocalDate.now();
        months = Months.monthsBetween(date1, date2).getMonths();

        // Print the result.
        System.out.println("Months between " +
                date1.toString("yyyy-MM-dd") + " and " +
                date2.toString("yyyy-MM-dd") + " = " +
                months + " month(s)");
    }
}

Here is the output of our program:

Months between 2021-01-01 and 2021-10-29 = 9 month(s)
Months between 2021-03-01 and 2021-10-29 = 7 month(s)

Maven Dependencies

<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.12.5</version>
</dependency>

The following code snippet show you how to get days between two dates. You can use the Days.daysBetween() method and pass the dates you want to calculate. This method return a Days object. To get the number in days you call the getDays() method.

package org.kodejava.joda;

import org.joda.time.DateMidnight;
import org.joda.time.Days;
import org.joda.time.LocalDate;

public class DaysBetweenDemo {
    public static void main(String[] args) {
        // Define the start and end dates. We use the DateMidnight
        // class to make sure that the calculation start from the
        // midnight.
        DateMidnight start = new DateMidnight("2021-07-01");
        DateMidnight end = new DateMidnight("2021-07-22");

        // Get days between the start date and end date.
        int days = Days.daysBetween(start, end).getDays();

        // Print the result.
        System.out.println("Days between " +
                start.toString("yyyy-MM-dd") + " and " +
                end.toString("yyyy-MM-dd") + " = " +
                days + " day(s)");

        // Using LocalDate object.
        LocalDate date1 = LocalDate.parse("2021-07-01");
        LocalDate date2 = LocalDate.now();
        days = Days.daysBetween(date1, date2).getDays();

        // Print the result.
        System.out.println("Days between " +
                date1.toString("yyyy-MM-dd") + " and " +
                date2.toString("yyyy-MM-dd") + " = " +
                days + " day(s)");
    }
}

Here is the result of the program:

Days between 2021-07-01 and 2021-07-22 = 21 day(s)
Days between 2021-07-01 and 2021-10-29 = 120 day(s)

To calculate differences between two date objects in Java 8 see the following example: How do I calculate difference between two dates?.

Maven Dependencies

<dependency>
    <groupId>joda-time</groupId>
    <artifactId>joda-time</artifactId>
    <version>2.12.7</version>
</dependency>