callback validation help

[eluser]dadamssg[/eluser]
im trying to write functions that check dates but im not sure how to use them in a validation script. I know you can use callbacks for your own functions but i don't know how to use it for this.

Code:

function date_exists($month,$day,$year)
    {
        if(checkdate($month,$day,$year))
          {  
            return TRUE;
          }
        else
          {
          return FALSE;
          }
    }

i need to use this twice...once for the start date and one for the end date. i can't see how i can do that using the $rules[]