| My understanding is that when you take the SAT if you have extended time, that colleges will not know that you took the SAT under extended time.
It should not affect you if you were diagnosed recently, as long as you have the chance to take the SAT.
You need to look up the rules with college board. I believe that there are strict guidelines that once the disability is diagnosed, that you have to have been in school and taking exams and tests in high school under extended testing conditions at your school for at least six months before you will be granted extended time on the SAT.
So if you school determined with outside testing that you fit the requirements to get extra time, they have to first award you extra time at school, and there has to be documentation that you took all exams and tests with extra time about six months from that diagnosis and taking the SAT.
I think if that is the case, then you may not get the benefit of getting extra time until October.
Check with your school and read the collegeboard extended time rules | 
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