Q:

How to set a timer to run when an app is both in foreground and in background?

I've my app set to keep receiving location updates while in background ("Background modes > Location updates"). My app listens for location updates while in foreground as well. But I need my app to keep listening for locations during a certain number of seconds I'm given, and to show a countdown if the app is in foreground, and to keep the time count if the app goes to background because I need to stop the location manager when the time ends.
I found no problems in setting an NSTimer when the app is in foreground, and showing a countdown, but I don't know how to handle it when app goes to background... in comments in AppDelegate's delegate applicationWillResignActive: and applicationDidEnterBackground: methods it is said that timers should be disabled and invalidated there... in addition, the timer I'm using is created in main thread:
[NSTimer scheduledTimerWithTimeInterval:1
                                 target:self
                               selector:@selector(updateCountdown:)
                               userInfo:nil
                                repeats:YES];

How could/should I manage this scenario?
Thanks in advance
EDIT: I'm going to try to explain an scenario I could have: my app listens for locations during a given time interval, and then it should stop listening. Let's say I'm going to give the user 5 min to reach certain location. User taps a kind of "start" button, and then I start the location manager to keep track of the user's positions, and I also start a countdown. When the countdown reaches 0, I stop the location manager and check the path the user has followed and the location he has reached. Since the app could go to background after the user has started the countdown, I have enabled Background modes > Location updates. Listening for locations while the app is in background is ok but, what if the countdown finishes while the app is still in background? I need to stop tracking user's positions even if the app is in background. I need a timer to keep running in background whatever countdown I need to set for the user, it could be 5 min, it could be 1 hour... 
Thanks again

A:

You can't run a timer in the background forever, it's not one of the background modes allowed by the OS so it will get terminated at some point.  Wrapping it in a background task will only buy you a few extra minutes (about 10) at the most.
To properly track time when your app enters the background you need to:
From applicationDidEnterBackground 

Save off a time stamp as soon as your app transitions to the background.
Terminate your timers

When your app re-enters the foreground:
From applicationWillEnterForeground edit or 'didUdateLocations'

Read the save time stamp
Use timeIntervalSinceDate to get the lapsed time in seconds
Restart your timer using the time span to set the remaining time

