Is disability proved by the date of diagnosis or the date when symptoms appeared?

I an a 45 y.o. woman who was recently diagnosed with multiple sclerosis, though I've had symptoms for at least 2=3 years. I was written up twice for being late/incomplete on quarterly assessments & am in danger of losing my job. 2 previous nurses were also consistently late/incomplete with these quarterly assessments, but were NOT discliplined to the best of my knowlege. One is of my own race & close in age. She received a transfere to another building within the company, which makes me think she was never written up as it is against company policy to allow transferes for anyone who is not ingood standing. The other nurse is of another race & younger (28 to 45) & is extremely close to the director of nurses, who wrote me up, so I doubt she was ever written up either. With both of these nurses, there were instanes where quarterly assessments hadn't been done for a year or more. Would disability protection begin when I was diagnosed, or when the symptoms actually began?