Start playing at 30 seconds in:
mediaPlayer.playMedia(media, ":start-time=30");
Play from the beginning until 60 seconds in:
mediaPlayer.playMedia(media, ":stop-time=60");
Play between 30 and 60 seconds in:
mediaPlayer.playMedia(media, ":start-time=30", ":stop-time=60");
You can also specify e.g. "30.5" if you need to specify fractions of a second.