Time Altering Wishes

So I was wondering, would an Eternal Dragon be able to grant a wish that alters the past? Also assuming such a wish could be granted, does the wish count when you loop back to the present day -OR- does the Dragon get to grant another wish when it loops back? Ex: Say Gohan wished that he had kept up with his training after the Cell Games.

I'll start the discussion with the 2nd question under the presumption that such a wish is grantable. So going with the example wish let's say Shenron could grant a wish. We loop back to present day and lo-and-behold Gohan has kept up with training. For some new reason, the Dragon Balls are gathered. Is Shenron able to grant a wish -OR- does he say something like your wish has already been granted when he's summoned?