title: iOS in-app purchase of auto-renewing subscriptions: how do we compute the renewal duration?
body: <p>In our iOS app, in-app purchase of an auto-renewable subscription provides access to certain additional features. Our server would like to check the in-app purchase receipt of each account to see if it expired or got renewed successfully. This is in order to decide to continue allowing access to those additional features (to that account user).</p>
  
  <p>On the server, how do we calculate the duration between renewals? Each month may have 28, 29, 30 or 31 days. So if somebody subscribes on Jan 29, does the app store make it up for renewal on Feb 29, or use some other algorithm, e.g., days from end of month (if in "second half"), so it might be Feb 26 or 27 (depending on whether Feb has 28 or 29 days this year)? We couldn't find the information about what algorithm Apple uses to calculate these durations. If somebody knows, or knows where to find it, could you let us know? Thanks!</p>