I agree with skatergirl...check all your clock and time settings. You could also try going to your calendar app and clearing its data, but before you do that, go to Settings, accounts&sync, Google, and turn off calendar sync. Then go and clear the data. Then go back and re-enable calendar sync.
Or you could try a different calendar app like the ICS app or Calendar Pad (free in market) or any of the other dozen apps. If they all do it, it's probably your phones clocks gone wonky, if not, its the app itself and clearing the data out of it and giving it a fresh start should do it.