Josinyc
if you cannot get a proof of employment, this is what i would do.
I would send in updated forms to Buffalo. So your date would now start from March 99. I would also write a letter to say what the difference between your old form and your new forms are and make a note that you took out that job because it fell outside you 10 year period for the updated form.
I think for CIC purpose date of application (or the way how people use that term) is different depending on the context. In your case, i would think it is the date of signing your form. Since your new form is signed in Mar 2009, it would go 10 years back.
As another example, for those wondering whether their application fell in the new process or old process (in Feb 2008), the date of application is probably when they received it at the visa office. So this example, I am just giving it because someone is going to read this post couple of days from now and going to take different meaning to the date of application. It depends on context.
MaceloS - If you are doing an updated form for Buffalo, I would could it from Feb 09 and make a note to say that you took out that job because it is irrelevant given the 10 year period that you are now looking at.
Of course if you can the letter of employment, it is always better to get one for that job, just because all what I say is just my opinion based on my logic. remember, i could be wrong in what i say above because i am not a CIC immigration officer. Merely stating what I would do.