```
  trigger SchedulePayment on Opportunity(after insert, after update) {
      if (Trigger.isInsert) {
          for (Opportunity o: Trigger.new) {
              Decimal expectedRevenue = o.ExpectedRevenue;
              Date paymentDate = o.Payment_Start_Date__c;
              Integer monthsBetweenStartAndEnd = o.Payment_Start_Date__c.monthsBetween(o.CloseDate);
              for (Integer i = 0; i < monthsBetweenStartAndEnd; i++) {
           // Create Payments when opportunity is created
                  Payment_Schedule__c payment = new Payment_Schedule__c();
                  payment.Amount__c = expectedRevenue / monthsBetweenStartAndEnd;
                  payment.Payment_Date__c = paymentDate;
                  payment.Opportunity__c = o.id;
                  insert payment;
                  // Assure we get the next month for the next payment record
                  paymentDate = paymentDate.addMonths(1);

              }
          }

      } else if (Trigger.isUpdate) {
          for (Opportunity o: trigger.new) {
              Decimal expectedRevenue = o.ExpectedRevenue;
              Date paymentDate = o.Payment_Start_Date__c;
              Date endDate = o.CloseDate;
              Integer monthsBetweenStartAndEnd = o.Payment_Start_Date__c.monthsBetween(o.CloseDate);
              // query related payment records.
              List < Payment_Schedule__c > payments = [select NAME, Payment_Date__c from Payment_Schedule__c where Opportunity__c != null and Opportunity__c in: trigger.new];

              Date lastScheduledPayment = payments.get(payments.Size() - 1).Payment_Date__c;
              Integer extraMonths = payments.Size() - monthsBetweenStartAndEnd;
              Integer monthsToAdd = monthsBetweenStartAndEnd - payments.Size();
              // this is to make sure we insert the correct months for new records on update
              boolean addMonths = true;
              for (Payment_Schedule__c payment: payments) {
               // remove records if time is shortened.
                  if (extraMonths > 0) {
                      delete payment;
                      extraMonths--;
                  }

                  // add records if time has been extended.
                  else if (extraMonths < 0) {
                                // this is to make sure we insert the correct months for new records on update
                      if (addMonths == true) {
                          paymentDate = paymentDate.addMonths(monthsToAdd);
                      }

                      Payment_Schedule__c newPayment = new Payment_Schedule__c();
                      newPayment.Amount__c = expectedRevenue / monthsBetweenStartAndEnd;
                      newPayment.Payment_Date__c = paymentDate;
                      newPayment.Opportunity__c = o.id;
                      insert newPayment;
                      addMonths = false;
                      paymentDate = paymentDate.addMonths(1);
                      // update the revenue of previous inserted records when a new record is inserted.
                      payment.Amount__c = expectedRevenue / monthsBetweenStartAndEnd;
                      update payment;


                  }
                  else if (extraMonths == 0) {
                      // update payment records.
                      payment.Payment_Date__c = paymentDate;
                      payment.Amount__c = expectedRevenue / monthsBetweenStartAndEnd;
                      update payment;
                      paymentDate = paymentDate.addMonths(1);

                  }

              }
          }


      }

  }

  }
```