Simply go to settings (the wrench icon).
It will display a screen with four options (Parental controls, Data management, internet settings and Other settings).
Go to Other settings.
Go to Date and Time.
Click current time, and viola you can change your 3DS clock, once this is changed the time will appear different in your Platnium game.
Source: My own knowledge, and I just did it.
But beware that in-game events will not occur for 24hours after the change.