これがドキュメントの2つのセッターです。
// The start time of movie playback. Defaults to NaN, indicating the natural start time of the movie.
@property(nonatomic) NSTimeInterval initialPlaybackTime;
// The end time of movie playback. Defaults to NaN, which indicates natural end time of the movie.
@property(nonatomic) NSTimeInterval endPlaybackTime;
MPMoviePlayerController *_theMovie;
[theMovie setInitialPlaybackTime:3.0]; //instead of [_theMovie setCurrentPlaybackTime:1.0];
[theMovie setEndPlaybackTime:6.0];
[_theMovie prepareToPlay];
[_theMovie play];
:)