This is not possible using server variables. We do support different server variables for date and time (as I think you already know them), however manipulation of these data is not possible.
You can however achieve this by writing a IS flow/java service that give the next file name as output. You can have your logic on the service to come up with the filename you want to rename to. This service output can be extracted in your event and can be used as server variable in subsequent actions, rename in your case.