x.m

- (void)setSeekTime:(int)inSeconds frame:(int)inFrame {    
    if (self.videoTrack) {
        float fps = [self.videoTrack nominalFrameRate];
        [self.playerItem seekToTime:CMTimeMake(inSeconds + inFrame / fps, 1)];
        // or: [self.playerItem seekToTime:CMTimeMake(inSeconds * fps + inFrame, fps)];
    } else {
        [self.playerItem seekToTime:CMTimeMakeWithSeconds(inSeconds, 1)];  
    }
}