by using a case statement and the datediff function...

Code:
case 
  when getdate() < convert(datetime, convert(varchar(4), year(getdate()) + '-09-01') then  convert(datetime, convert(varchar(4), (year(getdate())-1) + '-09-01') 
  else convert(datetime, convert(varchar(4), year(getdate()) + '-09-01') 
end as startDate
for the end date, it's similar logic... just change the 90-01 to 08-30 (I think you meant the end of august, not the end of September, otherwise you end up with an overlap.

-tg