One thing to keep in mind is that subtitles timings can vary based on the frames per second (FPS) of the video. For example, movies released in the US are encoded at 24 FPS while movies released in the UK (and some other parts of the world) are encoded at 25. Using a 25fps subtitle with a 24fps video (and vice versa) will cause the subtitles to become increasingly out of sync as the movie goes one.