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There is a strict time limit for background task , we only have less 10 minutes(from iOS7) to handle logic .
To register the app to run in the background , your app should be included in the following Categories .
Audio , VoIP , External Accessories and Bluetooth , Newsstand ,Location ,Fetch ,Remote Notifications .
Apple would never allow us to run a single Timer in the background .
As a workaround , I suggest you store the time(into file or something else) before going to background , and get the current time when opening it again , then you will easily get how many time it spent in background .
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