Programmatically reading and changing elapsed time of music(iPod's media library)?

I am creating a music application, much like the standard music app. Now, I am befuddled on how they were able to read and change the elapsed time of a song using a UISlider. How is it possible to read and change the elapsed time of a song?

By calling beginGeneratingPlaybackNotifications on the MPMusicPlayerController and registering for MPMusicPlayerControllerPlaybackStateDidChangeNotification notifications, you will receive periodic updates when the playback state has changed. In the handler for that notification you can then get the currentPlaybackTime from the MPMusicPlayerController.