Java Monthly Recurrence

Question

I am trying to get a monthly recurrence dates for a specific period. The code below works except when I am specifying the recurrence date of 30. Then it is generating the following error, which is normal:

Exception in thread "main" java.time.DateTimeException: Invalid date 'FEBRUARY 30'

My question is : Is there a way to set the recurrence to the last day of the month if the specified date (day) of the recurrence is higher than the last day of the month ?

Thanks !

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

        LocalDate startDate = LocalDate.of(2020, 10, 6);
        LocalDate endDate = LocalDate.of(2021, 12, 31);
        int dayOfMonth = 30;

        List<LocalDate> reportDates = getReportDates(startDate, endDate, dayOfMonth);
        System.out.println(reportDates);
    }

    private static List<LocalDate> getReportDates(LocalDate startDate, LocalDate endDate, int dayOfMonth) {
        List<LocalDate> dates = new ArrayList<>();
        LocalDate reportDate = startDate;
        
        while (reportDate.isBefore(endDate)) {
            reportDate = reportDate.plusMonths(1).withDayOfMonth(dayOfMonth);
            dates.add(reportDate);
        }
        return dates;
    }
    }

Answer 1

You need to specify a TemporalAdjuster to get the specific day of the month regardless of the month's length. It is passed as the third argument to your method.

Because I wasn't quite certain what you wanted, I included two adjusters. One for a specific day and one for the last day of the month. They can be used interchangeably. However, the latter will not reach Dec 31, 2021 since the before excludes ==

LocalDate startDate = LocalDate.of(2020, 10, 6);
LocalDate endDate = LocalDate.of(2021, 12, 31);

// Adjuster for a specific day
int dayOfMonth = 30;  // must be final or effectively final
                      // as it is used in the following lambda.
TemporalAdjuster specificDay = 
        date-> {
            LocalDate ld = (LocalDate)date;
            int max = ld.getMonth().length(ld.isLeapYear());
            int day = dayOfMonth > max ? max : dayOfMonth;
            return date.with(ChronoField.DAY_OF_MONTH,day);
};
        
        
// Adjuster for the lastDay of the month.
TemporalAdjuster lastDay = TemporalAdjusters.lastDayOfMonth();
    
List<LocalDate> reportDates =
        getReportDates(startDate, endDate, specificDay);

reportDates.forEach(System.out::println);
    
private static List<LocalDate> getReportDates(LocalDate startDate,
        LocalDate endDate, TemporalAdjuster specificDay) {
    List<LocalDate> dates = new ArrayList<>();
    
    // round up start day to specificDay
    LocalDate reportDate = startDate.with(specificDay);
    
    while (reportDate.isBefore(endDate)) {
        dates.add(reportDate);
        reportDate = reportDate.plusMonths(1).with(specificDay);
    }
    return dates;
}

Prints the following depending on choice of TemporalAdjuster

SpecificDay (30th)  LastDayOfMonth
    2020-10-30        2020-10-31
    2020-11-30        2020-11-30
    2020-12-30        2020-12-31
    2021-01-30        2021-01-31
    2021-02-28        2021-02-28
    2021-03-30        2021-03-31
    2021-04-30        2021-04-30
    2021-05-30        2021-05-31
    2021-06-30        2021-06-30
    2021-07-30        2021-07-31
    2021-08-30        2021-08-31
    2021-09-30        2021-09-30
    2021-10-30        2021-10-31
    2021-11-30        2021-11-30
    2021-12-30