We are working on an app right now and want to add a referral program in it. We are thinking a subscription based model and want to extend free trial period if user invite a friend and that friend will also get 7 days more on trial period. My question is if user already subscribed and first deduction will be on 1st June, how can we change it to 8th June after inviting friends ? And apple allows this ? Please share code to how to do this. Thanks.