LP Carryover and how it works

We are reducing the PTO max but we want to allow employees who are at their current max to be able to keep the hours that are over for 6 months. How can I accomplish that? If use the carryover tab and input a future date (6 months out) and I enter the hours limit, when I click on 'change' it gives me a message - 'carryover end date must be between entry and next carryover date' - but I can't find the dates they are referring to. This is my first time using LP, any help would be greatly appreciated.