You can use UILocalNotification instead of NSTimer. The limitations are that there is no actions in your application without your interaction, but there is triggered a notification even if the app is shutted down.

Apple documentation about UILocalNotifications:

Instances of UILocalNotification represent notifications that an application can schedule for presentation to its users at specific dates and times. The operating system is responsible for delivering the notification at the proper time; the application does not have to be running for this to happen.

...

When the system delivers a local notification, several things can happen, depending on the application state and the type of notification. If the application is not frontmost and visible, the system displays the alert message, badges the application, and plays a sound—whatever is specified in the notification. If the notification is an alert and the user taps the action button (or, if the device is locked, drags open the action slider), the application is launched. In the application:didFinishLaunchingWithOptions: method the application delegate can obtain the UILocalNotification object from the passed-in options dictionary by using the UIApplicationLaunchOptionsLocalNotificationKey key. The delegate can inspect the properties of the notification and, if the notification includes custom data in its userInfo dictionary, it can access that data and process it accordingly. On the other hand, if the local notification only badges the application icon, and the user in response launches the application, the application:didFinishLaunchingWithOptions: method is invoked, but no UILocalNotification object is included in the options dictionary.

If the application is foremost and visible when the system delivers the notification, no alert is shown, no icon is badged, and no sound is played. However, the application:didReceiveLocalNotification: is called if the application delegate implements it. The UILocalNotification instance is passed into this method, and the delegate can check its properties or access any custom data from the userInfo dictionary.

You can found a good tutorial here where the main idea is like this:

The notification setup:

Class cls = NSClassFromString(@"UILocalNotification");
if (cls != nil) {
    UILocalNotification *notif = [[cls alloc] init];
    notif.fireDate = [datePicker date];
    notif.timeZone = [NSTimeZone defaultTimeZone];

    notif.alertBody = TEXT;
    notif.alertAction = LAUNCH_BUTTON;
    notif.soundName = UILocalNotificationDefaultSoundName;
    notif.applicationIconBadgeNumber = NOTIFICATIONS_REMAINING;

    NSDictionary *userDict = [NSDictionary dictionaryWithObject:CUSTOM_INFO 
                                            forKey:kRemindMeNotificationDataKey];
    notif.userInfo = userDict;

    [[UIApplication sharedApplication] scheduleLocalNotification:notif];
    [notif release];
}

Handling the nofitication when the application is not running (in the applicaction delegate):

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    Class cls = NSClassFromString(@"UILocalNotification");
    if (cls) {
        UILocalNotification *notification = [launchOptions objectForKey:
                        UIApplicationLaunchOptionsLocalNotificationKey];

        if (notification) {
            NSString *reminderText = [notification.userInfo 
                        objectForKey:kRemindMeNotificationDataKey];
           [viewController showReminder:reminderText];
        }
    }

    application.applicationIconBadgeNumber = NOTIFICATIONS_REMAINING_LESS_ONE;

    [window addSubview:viewController.view];
    [window makeKeyAndVisible];

    return YES;
}

Application in the foreground (in the applicaction delegate):

- (void)application:(UIApplication *)application didReceiveLocalNotification:(UILocalNotification *)notification {

    UIApplicationState state = [application applicationState];
    if (state == UIApplicationStateInactive) {
        // Application was in the background when notification
        // was delivered.
    }
}

Are you sure a timer is what you want? Remember, a timer is only active when your app is active. It does not work when the app is inactive. If you re trying to get your app to do something at any time in the distant future, a timer will not really solve your problem because every time you quit the app, all its timers die.

Assuming that (1) you do just want to set timers for events while the app is active and (2) you always have an unknown number of events in the queue, then your biggest problem will be sorting out how to target an arbitrary number of timers for an arbitrary number of events.

Fortunately, timers can pass any arbitrary object in their userInfo property so you want to wrap every event in an object and pass that object to the timer. Then the timer fired method can extract the event and act on it.

Something like this:

// assume a data model with event objects with the attributes and methods shown

- (void) setTimerForEvent:(EventClass *) anEvent{
    [NSTimer timerWithTimeInterval:[anEvent eventTimeFromNow] 
                            target:self 
                          selector:@selector(fireEvent:) 
                          userInfo:anEvent 
                           repeats:NO];

}//------------------------------------setTimerForEvent:------------------------------------

- (void)fireEvent:(NSTimer*)theTimer{
    [self handleEvent:(EventClass *)theTimer.userInfo]; 
    [theTimer invalidate];
}//------------------------------------fireEvent:------------------------------------

The -[EventClass eventTimeFromNow] should return a NSTimerInterval of the seconds remaining between the time the method is called and time of the event.

You ll need a timer for each event. Maybe don t assign the timer to the timer variable. You can just create these timers and release them in the theFireEvent. You ll need to change that selector to @selector(theFireEvent) though, and make sure the method has the signature:

- (void)timerFireMethod:(NSTimer*)theTimer