Changes the number of days that this item represents.  This will move
        the end date the appropriate number of days away from the start date.
        The duration is calculated as the 1 plus the number of days from start
        to end, so a duration of 1 will have the same start and end date.  The
        duration needs to be a value greater than 0.
        
        :param      duration | <int>

def setDuration( self, duration ):
        """
        Changes the number of days that this item represents.  This will move
        the end date the appropriate number of days away from the start date.
        The duration is calculated as the 1 plus the number of days from start
        to end, so a duration of 1 will have the same start and end date.  The
        duration needs to be a value greater than 0.
        
        :param      duration | <int>
        """
        if ( duration <= 0 ):
            return
        
        self._dateEnd = self._dateStart.addDays(duration - 1)
        self.markForRebuild()