- (void)applicationDidEnterBackground:(UIApplication *)application {

    [NSTimer scheduledTimerWithTimeInterval:(5.0/5.0) target:self selector:@selector(check_expiry) userInfo:nil repeats:YES];

}

- (void)applicationDidEnterBackground:(UIApplication *)application {
    counter = YES;
    while (counter) {
        sleep(5);
        [self check_expiry];
    }
    // Counter is set to NO in willEnterForeground and didBecomeActive, but this loop continues to run due the sleep();
}