ZonedDateTime timezone inconsistency issue?

Question

I need to write one function which will create the future installments for the invoice. Below is the function which creates the list of future installment dates:-

public List<Date> getInstallmentDates(Invoice objectWithInvoiceDateField, int noOfInstallments, String instFreq)
{
    //objectWithInvoiceDateField.getInvoiceDate this will return java.util.Date instance
    ZonedDateTime invoiceDate = objectWithInvoiceDateField.getInvoiceDate.toInstant().atZone(ZoneId.systemDefault());
    ZonedDateTime firstInstallment = ZonedDateTime.of( invoiceDate.getYear(), invoiceDate.getMonthValue() , invoiceDate.getDayOfMonth() , 0 , 0 , 0, 0 , ZoneId.systemDefault());
    List<Date> installmentDates = new ArrayList();
    installmentDates.add(Date.from(firstInstallment.toInstant()));//First Installment
    /*Code for the subsequent installments*/
    for (int i = 1; i < noOfInstallments; i++) {
        ZonedDateTime subsequentInstallments = null;
        if(instFreq.equalsIgnoreCase("Quarterly")) {
            subsequentInstallments = firstInstallment.plusMonths(3*i);
        }
        else if(instFreq.equalsIgnoreCase("Semi-annual")){
            subsequentInstallments = firstInstallment.plusMonths(6*i);
        }
        else
            subsequentInstallments = firstInstallment.plusMonths(i);
        installmentDates.add(Date.from(subsequentInstallments.toInstant()));
    }
    return installmentDates;
}

This works as expected except for the last iteration. Below is the output if I run this method from main method for

getInstallmentDates(invoice, 5, "Monthly");

Thu Jul 30 00:00:00 EDT 2020
Sun Aug 30 00:00:00 EDT 2020
Wed Sep 30 00:00:00 EDT 2020
Fri Oct 30 00:00:00 EDT 2020
Mon Nov 30 00:00:00 ***EST*** 2020

Can some one please help me understand why the timezone for last instance is changed to EST? Thanks in advance!

Answer 1

Presumably because you have used the timezone to be ZoneId.systemDefault(), and your system defaults to a timezone that honours daylight saving time. Assuming EDT is Eastern Daylight Time and EST is Eastern Standard Time, in 2020 the end of daylight saving happens on 1 November and therefore the timezone name changes.