An invoice posted as year 2028 & then I made a credit of the same amount using year 2009. Now the customer is showing a credidt on their statement. I have cleared it from their account but of course their statement still shows the credit outstanding & it won't clear til 2028. How do I get rid of this problem? Can I change the due date on the credit somehow?

To answer your question, Due Dates cannot be edited for credit notes because they are not really ever due. Due Dates for Invoices can be changed, via an option in the Company Profile.

The Date Date is not the reason for the credit note and invoice not being cleared to history. The invoice dated in 2028 is - it and any matched transactions will not get cleared. You need to either run period end or print statements (however method you use for clearing transactions to history) and use a clearing date after 2028.

I would recommend using the Print Statements function to do this (clear transactions by Billing Cycle, create a dummy Billing Cycle and attach to this customer, and print statements only for that billing cycle) because then you can affect only that customer and nobody else.

When Printing Statements, you can choose to print 'Outstanding transactions only' - so the completed credit notes and invoices do not appear. This assumes you don't change the print date of statements, as other things kick in.

To prevent (or warn on) a transaction from being posted in such a future year, look in the Company Profile on the Integration tab for an option called 'Validate Closed Periods'. If a date gets entered in a batch for which a valid and open GL fiscal period exists, then a message will pop up. Setting this option to Disallow will not allow you to save a transaction with a date such as this.