Accurate timing in iOS

Question

I am looking at the 'Metronome' sample code from the iOS SDK (http://developer.apple.com/library/ios/#samplecode/Metronome/Introduction/Intro.html). I am running the metronome at 60 BPM, which means a tick every second. When I look at an external watch (the PC's watch), I see the metronome is running too slow - it misses about one beat each minute, which is app. 15msec of consistent error.