Instructions
Turn off your Nintendo DS system and remove the "Pokémon SoulSilver" cartridge.
Turn the system back on and let the main menu load.
Touch the small options button on the bottom of the screen. It looks like a small Nintendo DS.
Tap the blue calendar button, which is identifiable by its clock icon.
Touch the calendar icon to change the month, date and year, or the clock icon to change the time.
Exit the menu once your settings update, and allow the Nintendo DS to reboot.
Insert your "Pokémon SoulSilver" cartridge and reset the DS. Once your game loads, the in-game time will correspond to what you entered in the "time and date" menu.