Shredfingers

When using the "%period% string, auto update with dates option" in recurring invoices/orders, why doesn't it update one week at a time? In other words, I've entered four items and would expect each to be dated 8/19/11, 8/26/11, 9/2/11, 9/9/11 etc. But instead all four were dated 8/19/11. Is this not what this option does? If not, how can I make this happen for my recurring orders/invoices? I'm billing for weekly services monthly, bi-monthly, and quarterly and need to provide my customers with a date of service.