If you are playing on an emulator, you're probably out of luck. You'll need a GBA emu that supports the clock chip used in Ruby/Sapphire. However, try going back to Mishiro town and setting the clock to the time you want, then going back to where you need the time to be a certain value, without saving or powering down. That might well work.
If you're playing with a real cart, it's defective and you should get it replaced