It is not the fact that Rosh Hashanah is on a different date that is the problem it is that it is NOT in the fall. all feasts are to be celebrated in their proper season. this year Rosh Hashanah is scheduled outside fall outside it's season. every other time this means that the calendar is off. the reason for Adar II is to keep the feast in season the seasons are solar based while the jewish calendar is lunar based that is why to keep feasts in their season Adar II is sometimes needed.
 
Paul