You can now generate a monthly commission report for any prior completed month in Seller Admin > Pay Your Affiliates, where you would select the desired month and year, then click Check this month, then click where it says Click here to download the Mass Payment file (monthly commission report) for sales in that month.
You can also use your Transaction Log to generate a summary of total sales volume, revenue, and affiliate commission shares for any particular date range, product, and/or affiliate at any time. For instance, to pay out prior commission earnings for greater than a month at a time, you can view your log for the needed date range, then download that log view, open it in a spreadsheet program like Excel, and sort by the Affiliate ID column, which should make it relatively simple to sum up each affiliate's total earnings during the selected date range.
If you need to compose a
masspay.txt file to pay out older commissions, you can follow the format of your latest masspay file and/or refer to this help page explaining the PayPal Mass Payment file format.