I don't think you can. The timer starts when you do a new game and if youe xit and continue, the timer is gone. If I am wrong and there is a way, you could start a new game, do the first stage and on the second stage, click on the chess computer to get teleported to stage 08 (white room). But I fear the timer gets deactivated if you continue, load a save or warp. I have a save for doing it under 1h, but don't have one for under 30 mins.