A variety of issues have to come together in order for SSA to make a date of onset of disability decision. You have date of a head injury in 2002 but you worked (what you call off and on) until 3/09. Those are two dates to evaluate. Another date would be the dates of the medical evidence. You may have a progressive problem because of your head injury that has gotten worse over time. You may not have obtained significant amounts of medical treatment or thorough medical evaluation until you filed the claim in 12/09. You may not have supplied proof of the begin dates and end dates of all of your work so that six years of work could be determined to be an unsuccessful work attempt.

You should have gotten one notice that stated why 12/09 was chosen over 3/09, the last date you worked. And if you didn't supply proof that all your jobs met the criteria of unsuccessful work attempts, there is no way SSA can write off 6 years of work.