iphone - How to enforce time based license in an iPad app -

we're in process of completing application based on licensing system. users manage through web interface , can sync devices require. once device sync'd app self sufficient point user needs update device latest set of data.

now have implemented sync-process verify if user's license has expired or not before sync'ing. downside updates aren't required , more or less static on period of our licenses.

as such, we're trying figure out if possible on device itself, force user either sync our server @ regular intervals or when license supposed expire.

nsdate obvious no since values dependent on user can changed easily. mach_absolute_time() option considered timer changes when ipad restarted. need way track absolute time since license begun, regardless of whether application or device running or not , notify user of when app starts.

any appreciated.

interesting problem. keep nsuserdefaults value represents highest date app run detect if user has changed device clock. build in service client go out , check external server current time when network available. doing you're looking outside of user's control find true time.

if user off network won't time, these devices on network pretty often. trigger external time lookup when device has significant location change. capability described in apple documentation regarding multi-tasking options.