To change the system's time, load up the gamecube with the disk cover up. It will go to the system menu. One option wil say "Calendar" Click on that and it will bring you to the time on your system. If you want to change the Month, click on the month number and then you will be able to set the month. Click on the date number and then you can change the date.

P.S: I tried the cheat you want to do; it DOES work. Oh, and by the way, you should click on the time number and set it to 0:00:00 whenever you want to do the cheat. Once I didn't do it and while the game was loading up, the date changed.