If you compile app with
develop certificate, app will stop at the date when certificate expire.
If you compile app with
distribution certificate and publish app on Apple Store, no matter when certificate expire, app will work.
I have app 2 years old with expired certificate on store. App still work. App still can be downloaded.
Here is one of app from
September 2015 made for one of my customers
link. Certificate is expired on September 2016, and you can still download app and use it.
All app on store not updated in last year have expired certificate.