[otrs] Time Accounting: How to make months start on 15th day instead of 1st Day?

Hello, i have a question regarding the OTRS Time Accounting Module. How is it possible to make the module reset all times on 15th of each month instead of 1st Day of a month? We track times for a customer project and the payment cycle ist he 15th of each month. Thus we should be able to track from 15th of a month, not 1st day. Can anyone give me a small hint how to realize that? Thanks so much in advance. Daniel