Q:

Non-Renewing Subscription: turn back time

I'm creating an app that will unlock features to users for a limited time. Premium features let's call them. They can buy a subscription for a month, 6 months or a year. These subscriptions would be non-renewing. After a lot of searching the web and finding very different "solutions", I turn to StackOverflow for what the best should be.
When a subscription is bought, the end date is stored locally on the device until synced via iCloud, because Apple wants you to have this subscription on all your devices. The app checks iCloud to see if the current subscription is still available, shows a nice countdown for the users with days left. But I'm currently encountering a problem. When the user has paid for a month, and changes the date of the device to a year earlier (or something else), it can use the subscription for a year. The countdown now doesn't show 23 days left, but 389.
How can I avoid this?
Should I check a certain server to check for the real time and date?
What if a user only uses the app in airplane mode?
Thx in advance!

A:

You could check standard web-based time-servers to do this. Since I have my own server (as part my in-app purchase mechanisms), I just ask my server for the current time, at the server. I do this every time the app launches, and every time I make a server-call (usually when in-app purchases are done). I then store the time locally (either in the KeyChain or NSUserDefaults, can't remember which!), and use it (and the device time, which ever is later in time) to compute purchase expiries.
Of course, this assumes that the user has a network connection when they launch the app. You can only do so much though!

