Joda Time: Calculate time to next full interval in month

Question

I am implementing an application where the user can create tasks with due dates and repeat them e.g. repeat every x days, every x weeks, every x months etc.

When a task is due I want to calculate the next (repeated) due date based on the defined interval. My implementation for weeks works but not for months, because I cannot convert months to minutes.

AlarmHelper.java

// Works
static LocalDateTime getNextWeeklyIntervalDate(LocalDateTime creationTime, LocalDateTime dueTime, int intervalInWeeks) {
    int intervalInMinutes = Weeks.weeks(intervalInWeeks).toStandardMinutes().getMinutes();
    int timePassed = Minutes.minutesBetween(creationTime, dueTime).getMinutes();
    int rest = timePassed % intervalInMinutes;
    int minutesToNextFullInterval = intervalInMinutes - rest;
    return dueTime.plusMinutes(minutesToNextFullInterval );
}

// How to implement this analogous to weekly, but for monthly intervals? toStandardMinutes() does not exist for months  
static LocalDateTime getNextMonthlyIntervalDate(LocalDateTime creationTime, LocalDateTime dueTime, int intervalInMonth) {
    int intervalInMinutes = Months.months(intervalInMonth).toStandardMinutes().getMinutes(); // toStandardMinutes() does not exist
}

AlarmHelperTest.java

// User created the task on 01.01.2021 (January 1st, 2021) and want to repeat it weekly.
// So the intervals are 08.01.2021, 15.01.2021 etc.
// In the test the task is due on 12.01.2021, which means the next weekly interval is fulfilled on 15.01.2021.

//The test passes
 @Test
 public void TestGetNextWeeklyIntervalDate() {
    LocalDateTime creation = createLocalDateTime("01.01.2021 06:30:00");
    LocalDateTime due = createLocalDateTime("12.01.2021 05:30:00");
    LocalDateTime expected = createLocalDateTime("15.01.2021 06:30:00");
    LocalDateTime result = AlarmHelper.getNextWeeklyIntervalDate(creation, due, 1);
    assertEquals(expected, result);
}

Answer 1

Note: Check the following notice at the Home Page of Joda-Time

Joda-Time is the de facto standard date and time library for Java prior to Java SE 8. Users are now asked to migrate to java.time (JSR-310).

Therefore, I recommend you do it with the java.time API using the following methods:

1. LocalDateTime#isBefore
2. LocalDateTime#plusWeeks
3. LocalDateTime#plusMonths

Using the java.time API:

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        LocalDateTime creation = createLocalDateTime("01.01.2021 06:30:00");
        LocalDateTime due = createLocalDateTime("12.01.2021 05:30:00");
        System.out.println(getNextWeeklyIntervalDate(creation, due, 1));
        System.out.println(getNextMonthlyIntervalDate(creation, due, 1));
    }

    static LocalDateTime getNextWeeklyIntervalDate(LocalDateTime creationTime, LocalDateTime dueTime,
            int intervalInWeeks) {
        LocalDateTime ldt = creationTime;
        while (ldt.isBefore(dueTime)) {
            ldt = ldt.plusWeeks(intervalInWeeks);
        }
        return ldt;
    }

    static LocalDateTime getNextMonthlyIntervalDate(LocalDateTime creationTime, LocalDateTime dueTime,
            int intervalInMonth) {
        LocalDateTime ldt = creationTime;
        while (ldt.isBefore(dueTime)) {
            ldt = ldt.plusMonths(intervalInMonth);
        }
        return ldt;
    }

    static LocalDateTime createLocalDateTime(String dateTimeStr) {
        return LocalDateTime.parse(dateTimeStr, DateTimeFormatter.ofPattern("dd.MM.yyyy HH:mm:ss", Locale.ENGLISH));
    }
}

Output:

2021-01-15T06:30
2021-02-01T06:30

These methods are available in Joda-time API as well i.e. all you need to change is the way how the date-time string is parsed and LocalDateTime is obtained.

Using Joda-time API:

import org.joda.time.LocalDateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

public class Main {
    public static void main(String[] args) {
        LocalDateTime creation = createLocalDateTime("01.01.2021 06:30:00");
        LocalDateTime due = createLocalDateTime("12.01.2021 05:30:00");
        System.out.println(getNextWeeklyIntervalDate(creation, due, 1));
        System.out.println(getNextMonthlyIntervalDate(creation, due, 1));
    }

    static LocalDateTime getNextWeeklyIntervalDate(LocalDateTime creationTime, LocalDateTime dueTime,
            int intervalInWeeks) {
        LocalDateTime ldt = creationTime;
        while (ldt.isBefore(dueTime)) {
            ldt = ldt.plusWeeks(intervalInWeeks);
        }
        return ldt;
    }

    static LocalDateTime getNextMonthlyIntervalDate(LocalDateTime creationTime, LocalDateTime dueTime,
            int intervalInMonth) {
        LocalDateTime ldt = creationTime;
        while (ldt.isBefore(dueTime)) {
            ldt = ldt.plusMonths(intervalInMonth);
        }
        return ldt;
    }

    static LocalDateTime createLocalDateTime(String dateTimeStr) {
        DateTimeFormatter dtf = DateTimeFormat.forPattern("dd.MM.yyyy HH:mm:ss");
        LocalDateTime ldt = dtf.parseDateTime(dateTimeStr).toLocalDateTime();
        return ldt;
    }
}

Output:

2021-01-15T06:30:00.000
2021-02-01T06:30:00.000

As you can see, every thing, except the code inside createLocalDateTime, is same for both java.time and Joda-time API.